Allow the planner's estimate of the fraction of a cursor's rows that will be

[postgresql] / doc / src / sgml / config.sgml

<!-- $PostgreSQL: pgsql/doc/src/sgml/config.sgml,v 1.177 2008/05/02 21:26:09 tgl Exp $ -->

<chapter Id="runtime-config">
  <title>Server Configuration</title>

  <indexterm>
   <primary>configuration</primary>
   <secondary>of the server</secondary>
  </indexterm>

  <para>
   There are many configuration parameters that affect the behavior of
   the database system. In the first section of this chapter, we
   describe how to set configuration parameters. The subsequent sections
   discuss each parameter in detail.
  </para>

  <sect1 id="config-setting">
   <title>Setting Parameters</title>

   <para>
    All parameter names are case-insensitive. Every parameter takes a
    value of one of four types: Boolean, integer, floating point,
    or string. Boolean values can be written as <literal>ON</literal>,
    <literal>OFF</literal>, <literal>TRUE</literal>,
    <literal>FALSE</literal>, <literal>YES</literal>,
    <literal>NO</literal>, <literal>1</literal>, <literal>0</literal>
    (all case-insensitive) or any unambiguous prefix of these.
   </para>

   <para>
    Some settings specify a memory or time value.  Each of these has an
    implicit unit, which is either kilobytes, blocks (typically eight
    kilobytes), milliseconds, seconds, or minutes.  Default units can be
    queried by referencing <literal>pg_settings.unit</>. For convenience,
    a different unit can also be specified explicitly.  Valid memory units
    are <literal>kB</literal> (kilobytes), <literal>MB</literal>
    (megabytes), and <literal>GB</literal> (gigabytes); valid time units
    are <literal>ms</literal> (milliseconds), <literal>s</literal>
    (seconds), <literal>min</literal> (minutes), <literal>h</literal>
    (hours), and <literal>d</literal> (days).  Note that the multiplier
    for memory units is 1024, not 1000.
   </para>

   <para>
    One way to set these parameters is to edit the file
    <filename>postgresql.conf</><indexterm><primary>postgresql.conf</></>,
    which is normally kept in the data directory. (<application>initdb</>
    installs a default copy there.) An example of what this file might look
    like is:
<programlisting>
# This is a comment
log_connections = yes
log_destination = 'syslog'
search_path = '"$user", public'
shared_buffers = 128MB
</programlisting>
    One parameter is specified per line. The equal sign between name and
    value is optional. Whitespace is insignificant and blank lines are
    ignored. Hash marks (<literal>#</literal>) introduce comments
    anywhere.  Parameter values that are not simple identifiers or
    numbers must be single-quoted.  To embed a single quote in a parameter
    value, write either two quotes (preferred) or backslash-quote.
   </para>

   <para>
    <indexterm>
     <primary><literal>include</></primary>
     <secondary>in configuration file</secondary>
    </indexterm>
    In addition to parameter settings, the <filename>postgresql.conf</>
    file can contain <firstterm>include directives</>, which specify
    another file to read and process as if it were inserted into the
    configuration file at this point.  Include directives simply look like:
<programlisting>
include 'filename'
</programlisting>
    If the file name is not an absolute path, it is taken as relative to
    the directory containing the referencing configuration file.
    Inclusions can be nested.
   </para>

   <para>
    <indexterm>
     <primary>SIGHUP</primary>
    </indexterm>
    The configuration file is reread whenever the main server process receives a
    <systemitem>SIGHUP</> signal (which is most easily sent by means
    of <literal>pg_ctl reload</>). The main server process
    also propagates this signal to all currently running server
    processes so that existing sessions also get the new
    value. Alternatively, you can send the signal to a single server
    process directly.  Some parameters can only be set at server start;
    any changes to their entries in the configuration file will be ignored
    until the server is restarted.
   </para>

   <para>
    A second way to set these configuration parameters is to give them
    as a command-line option to the <command>postgres</command> command, such as:
<programlisting>
postgres -c log_connections=yes -c log_destination='syslog'
</programlisting>
    Command-line options override any conflicting settings in
    <filename>postgresql.conf</filename>.  Note that this means you won't
    be able to change the value on-the-fly by editing
    <filename>postgresql.conf</filename>, so while the command-line
    method might be convenient, it can cost you flexibility later.
   </para>

   <para>
    Occasionally it is useful to give a command line option to
    one particular session only. The environment variable
    <envar>PGOPTIONS</envar> can be used for this purpose on the
    client side:
<programlisting>
env PGOPTIONS='-c geqo=off' psql
</programlisting>
    (This works for any <application>libpq</>-based client application, not
    just <application>psql</application>.) Note that this won't work for
    parameters that are fixed when the server is started or that must be
    specified in <filename>postgresql.conf</filename>.
   </para>

   <para>
    Furthermore, it is possible to assign a set of parameter settings to
    a user or a database.  Whenever a session is started, the default
    settings for the user and database involved are loaded.  The
    commands <xref linkend="sql-alteruser" endterm="sql-alteruser-title">
    and <xref linkend="sql-alterdatabase" endterm="sql-alterdatabase-title">,
    respectively, are used to configure these settings.  Per-database
    settings override anything received from the
    <command>postgres</command> command-line or the configuration
    file, and in turn are overridden by per-user settings; both are
    overridden by per-session settings.
   </para>

   <para>
    Some parameters can be changed in individual <acronym>SQL</acronym>
    sessions with the <xref linkend="SQL-SET" endterm="SQL-SET-title">
    command, for example:
<screen>
SET ENABLE_SEQSCAN TO OFF;
</screen>
    If <command>SET</> is allowed, it overrides all other sources of
    values for the parameter. Some parameters cannot be changed via
    <command>SET</command>: for example, if they control behavior that
    cannot be changed without restarting the entire
    <productname>PostgreSQL</productname> server.  Also, some parameters can
    be modified via <command>SET</command> or <command>ALTER</> by superusers,
    but not by ordinary users.
   </para>

   <para>
    The <xref linkend="SQL-SHOW" endterm="SQL-SHOW-title">
    command allows inspection of the current values of all parameters.
   </para>

   <para>
    The virtual table <structname>pg_settings</structname>
    (described in <xref linkend="view-pg-settings">) also allows
    displaying and updating session run-time parameters.  It is equivalent
    to <command>SHOW</> and <command>SET</>, but can be more convenient
    to use because it can be joined with other tables, or selected from using
    any desired selection condition. It also contains more information about
    what values are allowed for the parameters.
   </para>
  </sect1>

   <sect1 id="runtime-config-file-locations">
    <title>File Locations</title>

     <para>
      In addition to the <filename>postgresql.conf</filename> file
      already mentioned, <productname>PostgreSQL</productname> uses
      two other manually-edited configuration files, which control
      client authentication (their use is discussed in <xref
      linkend="client-authentication">).  By default, all three
      configuration files are stored in the database cluster's data
      directory.  The parameters described in this section allow the
      configuration files to be placed elsewhere.  (Doing so can ease
      administration.  In particular it is often easier to ensure that
      the configuration files are properly backed-up when they are
      kept separate.)
     </para>

     <variablelist>
     <varlistentry id="guc-data-directory" xreflabel="data_directory">
      <term><varname>data_directory</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>data_directory</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the directory to use for data storage.
         This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-config-file" xreflabel="config_file">
      <term><varname>config_file</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>config_file</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the main server configuration file
         (customarily called <filename>postgresql.conf</>).
         This parameter can only be set on the <command>postgres</command> command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-hba-file" xreflabel="hba_file">
      <term><varname>hba_file</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>hba_file</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the configuration file for host-based authentication
         (customarily called <filename>pg_hba.conf</>).
         This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ident-file" xreflabel="ident_file">
      <term><varname>ident_file</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>ident_file</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the configuration file for
         <application>ident</> authentication
         (customarily called <filename>pg_ident.conf</>).
         This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-external-pid-file" xreflabel="external_pid_file">
      <term><varname>external_pid_file</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>external_pid_file</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the name of an additional process-id (PID) file that the
        server should create for use by server administration programs.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     </variablelist>

     <para>
      In a default installation, none of the above parameters are set
      explicitly.  Instead, the
      data directory is specified by the <option>-D</option> command-line
      option or the <envar>PGDATA</envar> environment variable, and the
      configuration files are all found within the data directory.
     </para>

     <para>
      If you wish to keep the configuration files elsewhere than the
      data directory, the <command>postgres</command> <option>-D</option>
      command-line option or <envar>PGDATA</envar> environment variable
      must point to the directory containing the configuration files,
      and the <varname>data_directory</> parameter must be set in
      <filename>postgresql.conf</filename> (or on the command line) to show
      where the data directory is actually located.  Notice that
      <varname>data_directory</> overrides <option>-D</option> and
      <envar>PGDATA</envar> for the location
      of the data directory, but not for the location of the configuration
      files.
     </para>

     <para>
      If you wish, you can specify the configuration file names and locations
      individually using the parameters <varname>config_file</>,
      <varname>hba_file</> and/or <varname>ident_file</>.
      <varname>config_file</> can only be specified on the
      <command>postgres</command> command line, but the others can be
      set within the main configuration file.  If all three parameters plus
      <varname>data_directory</> are explicitly set, then it is not necessary
      to specify <option>-D</option> or <envar>PGDATA</envar>.
     </para>

     <para>
      When setting any of these parameters, a relative path will be interpreted
      with respect to the directory in which <command>postgres</command>
      is started.
     </para>
   </sect1>

   <sect1 id="runtime-config-connection">
    <title>Connections and Authentication</title>

    <sect2 id="runtime-config-connection-settings">
     <title>Connection Settings</title>

     <variablelist>

     <varlistentry id="guc-listen-addresses" xreflabel="listen_addresses">
      <term><varname>listen_addresses</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>listen_addresses</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         Specifies the TCP/IP address(es) on which the server is
         to listen for connections from client applications.
         The value takes the form of a comma-separated list of host names
         and/or numeric IP addresses.  The special entry <literal>*</>
         corresponds to all available IP interfaces.
         If the list is empty, the server does not listen on any IP interface
         at all, in which case only Unix-domain sockets can be used to connect
         to it.
         The default value is <systemitem class="systemname">localhost</>,
         which allows only local <quote>loopback</> connections to be made.
         This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-port" xreflabel="port">
      <term><varname>port</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>port</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The TCP port the server listens on; 5432 by default.  Note that the
        same port number is used for all IP addresses the server listens on.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-connections" xreflabel="max_connections">
      <term><varname>max_connections</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_connections</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Determines the maximum number of concurrent connections to the
        database server. The default is typically 100 connections, but
        might be less if your kernel settings will not support it (as
        determined during <application>initdb</>).  This parameter can
        only be set at server start.
       </para>

       <para>
        Increasing this parameter might cause <productname>PostgreSQL</>
        to request more <systemitem class="osname">System V</> shared
        memory or semaphores than your operating system's default configuration
        allows. See <xref linkend="sysvipc"> for information on how to
        adjust those parameters, if necessary.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-superuser-reserved-connections"
     xreflabel="superuser_reserved_connections">
      <term><varname>superuser_reserved_connections</varname>
      (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>superuser_reserved_connections</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Determines the number of connection <quote>slots</quote> that
        are reserved for connections by <productname>PostgreSQL</>
        superusers.  At most <xref linkend="guc-max-connections">
        connections can ever be active simultaneously.  Whenever the
        number of active concurrent connections is at least
        <varname>max_connections</> minus
        <varname>superuser_reserved_connections</varname>, new
        connections will be accepted only for superusers.
       </para>

       <para>
        The default value is three connections. The value must be less
        than the value of <varname>max_connections</varname>. This
        parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-unix-socket-directory" xreflabel="unix_socket_directory">
      <term><varname>unix_socket_directory</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>unix_socket_directory</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the directory of the Unix-domain socket on which the
        server is to listen for
        connections from client applications.  The default is normally
        <filename>/tmp</filename>, but can be changed at build time.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-unix-socket-group" xreflabel="unix_socket_group">
      <term><varname>unix_socket_group</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>unix_socket_group</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the owning group of the Unix-domain socket.  (The owning
        user of the socket is always the user that starts the
        server.)  In combination with the parameter
        <varname>unix_socket_permissions</varname> this can be used as
        an additional access control mechanism for Unix-domain connections.
        By default this is the empty string, which selects the default
        group for the current user.  This parameter can only be set at
        server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-unix-socket-permissions" xreflabel="unix_socket_permissions">
      <term><varname>unix_socket_permissions</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>unix_socket_permissions</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the access permissions of the Unix-domain socket.  Unix-domain
        sockets use the usual Unix file system permission set.
        The parameter value is expected to be a numeric mode
        specification in the form accepted by the
        <function>chmod</function> and <function>umask</function>
        system calls.  (To use the customary octal format the number
        must start with a <literal>0</literal> (zero).)
       </para>

       <para>
        The default permissions are <literal>0777</literal>, meaning
        anyone can connect. Reasonable alternatives are
        <literal>0770</literal> (only user and group, see also
        <varname>unix_socket_group</varname>) and <literal>0700</literal>
        (only user). (Note that for a Unix-domain socket, only write
        permission matters and so there is no point in setting or revoking
        read or execute permissions.)
       </para>

       <para>
        This access control mechanism is independent of the one
        described in <xref linkend="client-authentication">.
       </para>

       <para>
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-bonjour-name" xreflabel="bonjour_name">
      <term><varname>bonjour_name</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>bonjour_name</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the <productname>Bonjour</productname> broadcast
        name.  The computer name is used if this parameter is set to the
        empty string <literal>''</> (which is the default).  This parameter is
        ignored if the server was not compiled with
        <productname>Bonjour</productname> support.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-tcp-keepalives-idle" xreflabel="tcp_keepalives_idle">
      <term><varname>tcp_keepalives_idle</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>tcp_keepalives_idle</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        On systems that support the <symbol>TCP_KEEPIDLE</symbol> socket option, specifies the
        number of seconds between sending keepalives on an otherwise idle
        connection. A value of zero uses the system default. If <symbol>TCP_KEEPIDLE</symbol> is
        not supported, this parameter must be zero. This parameter is ignored for
        connections made via a Unix-domain socket.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-tcp-keepalives-interval" xreflabel="tcp_keepalives_interval">
      <term><varname>tcp_keepalives_interval</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>tcp_keepalives_interval</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        On systems that support the <symbol>TCP_KEEPINTVL</symbol> socket option, specifies how
        long, in seconds, to wait for a response to a keepalive before
        retransmitting. A value of zero uses the system default. If <symbol>TCP_KEEPINTVL</symbol>
        is not supported, this parameter must be zero. This parameter is ignored
        for connections made via a Unix-domain socket.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-tcp-keepalives-count" xreflabel="tcp_keepalives_count">
      <term><varname>tcp_keepalives_count</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>tcp_keepalives_count</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        On systems that support the <symbol>TCP_KEEPCNT</symbol> socket option, specifies how
        many keepalives can be lost before the connection is considered dead.
        A value of zero uses the system default. If <symbol>TCP_KEEPCNT</symbol> is not
        supported, this parameter must be zero. This parameter is ignored
        for connections made via a Unix-domain socket.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>
     <sect2 id="runtime-config-connection-security">
     <title>Security and Authentication</title>

     <variablelist>
     <varlistentry id="guc-authentication-timeout" xreflabel="authentication_timeout">
      <term><varname>authentication_timeout</varname> (<type>integer</type>)</term>
      <indexterm><primary>timeout</><secondary>client authentication</></indexterm>
      <indexterm><primary>client authentication</><secondary>timeout during</></indexterm>
      <indexterm>
       <primary><varname>authentication_timeout</> configuration parameter</primary>
      </indexterm>

      <listitem>
       <para>
        Maximum time to complete client authentication, in seconds. If a
        would-be client has not completed the authentication protocol in
        this much time, the server breaks the connection. This prevents
        hung clients from occupying a connection indefinitely.
        The default is one minute (<literal>1m</>).
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-ssl" xreflabel="ssl">
      <term><varname>ssl</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>ssl</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables <acronym>SSL</> connections. Please read
        <xref linkend="ssl-tcp"> before using this. The default
        is <literal>off</>. This parameter can only be set at server
        start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ssl-ciphers" xreflabel="ssl_ciphers">
      <term><varname>ssl_ciphers</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>ssl_ciphers</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies a list of <acronym>SSL</> ciphers that are allowed to be
        used on secure connections. See the <application>openssl</>
        manual page for a list of supported ciphers.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-password-encryption" xreflabel="password_encryption">
      <term><varname>password_encryption</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>password_encryption</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When a password is specified in <xref
        linkend="sql-createuser" endterm="sql-createuser-title"> or
        <xref linkend="sql-alteruser" endterm="sql-alteruser-title">
        without writing either <literal>ENCRYPTED</> or
        <literal>UNENCRYPTED</>, this parameter determines whether the
        password is to be encrypted. The default is <literal>on</>
        (encrypt the password).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-krb-realm" xreflabel="krb_realm">
      <term><varname>krb_realm</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>krb_realm</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the realm to match Kerberos, GSSAPI and SSPI user names against.
        See <xref linkend="kerberos-auth">, <xref linkend="gssapi-auth"> or
        <xref linkend="sspi-auth"> for details. This parameter can only be
        set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
      <term><varname>krb_server_keyfile</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>krb_server_keyfile</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the location of the Kerberos server key file. See
        <xref linkend="kerberos-auth"> or <xref linkend="gssapi-auth"> 
        for details. This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-krb-srvname" xreflabel="krb_srvname">
      <term><varname>krb_srvname</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>krb_srvname</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the Kerberos service name. See <xref linkend="kerberos-auth">
        for details.  This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-krb-server-hostname" xreflabel="krb_server_hostname">
      <term><varname>krb_server_hostname</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>krb_server_hostname</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the host name part of the service principal.
        This, combined with <varname>krb_srvname</>, is used to generate
        the complete service principal, that is
        <varname>krb_srvname</><literal>/</><varname>krb_server_hostname</><literal>@</>REALM.
        If not set, the default is the server host name.  See <xref linkend="kerberos-auth">
        for details.  This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-krb-caseins-users" xreflabel="krb_caseins_users">
      <term><varname>krb_caseins_users</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>krb_caseins_users</varname> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets whether Kerberos and GSSAPI user names should be treated
        case-insensitively.
        The default is <literal>off</> (case sensitive). This parameter
        can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-db-user-namespace" xreflabel="db_user_namespace">
      <term><varname>db_user_namespace</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>db_user_namespace</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This parameter enables per-database user names.  It is off by default.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>

       <para>
        If this is on, you should create users as <literal>username@dbname</>.
        When <literal>username</> is passed by a connecting client,
        <literal>@</> and the database name are appended to the user
        name and that database-specific user name is looked up by the
        server. Note that when you create users with names containing
        <literal>@</> within the SQL environment, you will need to
        quote the user name.
       </para>

       <para>
        With this parameter enabled, you can still create ordinary global
        users.  Simply append <literal>@</> when specifying the user
        name in the client.  The <literal>@</> will be stripped off
        before the user name is looked up by the server.
       </para>

       <note>
        <para>
         This feature is intended as a temporary measure until a
         complete solution is found.  At that time, this option will
         be removed.
        </para>
       </note>
      </listitem>
     </varlistentry>

    </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-resource">
    <title>Resource Consumption</title>

    <sect2 id="runtime-config-resource-memory">
     <title>Memory</title>

     <variablelist>
     <varlistentry id="guc-shared-buffers" xreflabel="shared_buffers">
      <term><varname>shared_buffers</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>shared_buffers</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the amount of memory the database server uses for shared
        memory buffers.  The default is typically 32 megabytes
        (<literal>32MB</>), but might be less if your kernel settings will
        not support it (as determined during <application>initdb</>).
        This setting must be at least 128 kilobytes.  (Non-default
        values of <symbol>BLCKSZ</symbol> change the minimum.)  However,
        settings significantly higher than the minimum are usually needed
        for good performance.  Several tens of megabytes are recommended
        for production installations.  This parameter can only be set at
        server start.
       </para>

       <para>
        Increasing this parameter might cause <productname>PostgreSQL</>
        to request more <systemitem class="osname">System V</> shared
        memory than your operating system's default configuration
        allows. See <xref linkend="sysvipc"> for information on how to
        adjust those parameters, if necessary.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-temp-buffers" xreflabel="temp_buffers">
      <term><varname>temp_buffers</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>temp_buffers</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the maximum number of temporary buffers used by each database
        session.  These are session-local buffers used only for access to
        temporary tables.  The default is eight megabytes
        (<literal>8MB</>).  The setting can be changed within individual
        sessions, but only up until the first use of temporary tables
        within a session; subsequent attempts to change the value will
        have no effect on that session.
       </para>

       <para>
        A session will allocate temporary buffers as needed up to the limit
        given by <varname>temp_buffers</>.  The cost of setting a large
        value in sessions that do not actually need a lot of temporary
        buffers is only a buffer descriptor, or about 64 bytes, per
        increment in <varname>temp_buffers</>.  However if a buffer is
        actually used an additional 8192 bytes will be consumed for it
        (or in general, <symbol>BLCKSZ</symbol> bytes).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-prepared-transactions" xreflabel="max_prepared_transactions">
      <term><varname>max_prepared_transactions</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_prepared_transactions</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the maximum number of transactions that can be in the
        <quote>prepared</> state simultaneously (see <xref
        linkend="sql-prepare-transaction"
        endterm="sql-prepare-transaction-title">).
        Setting this parameter to zero disables the prepared-transaction
        feature.
        The default is five transactions.
        This parameter can only be set at server start.
       </para>

       <para>
        If you are not using prepared transactions, this parameter may as
        well be set to zero.  If you are using them, you will probably
        want <varname>max_prepared_transactions</varname> to be at least
        as large as <xref linkend="guc-max-connections">, to avoid unwanted
        failures at the prepare step.
       </para>

       <para>
        Increasing this parameter might cause <productname>PostgreSQL</>
        to request more <systemitem class="osname">System V</> shared
        memory than your operating system's default configuration
        allows. See <xref linkend="sysvipc"> for information on how to
        adjust those parameters, if necessary.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-work-mem" xreflabel="work_mem">
      <term><varname>work_mem</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>work_mem</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the amount of memory to be used by internal sort operations
        and hash tables before switching to temporary disk files. The value
        defaults to one megabyte (<literal>1MB</>).
        Note that for a complex query, several sort or hash operations might be
        running in parallel; each one will be allowed to use as much memory
        as this value specifies before it starts to put data into temporary
        files. Also, several running sessions could be doing such operations
        concurrently.  So the total memory used could be many
        times the value of <varname>work_mem</varname>; it is necessary to
        keep this fact in mind when choosing the value. Sort operations are
        used for <literal>ORDER BY</>, <literal>DISTINCT</>, and
        merge joins.
        Hash tables are used in hash joins, hash-based aggregation, and
        hash-based processing of <literal>IN</> subqueries.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-maintenance-work-mem" xreflabel="maintenance_work_mem">
      <term><varname>maintenance_work_mem</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>maintenance_work_mem</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the maximum amount of memory to be used in maintenance
        operations, such as <command>VACUUM</command>, <command>CREATE
        INDEX</>, and <command>ALTER TABLE ADD FOREIGN KEY</>.  It defaults
        to 16 megabytes (<literal>16MB</>).  Since only one of these
        operations can be executed at a time by a database session, and
        an installation normally doesn't have many of them running
        concurrently, it's safe to set this value significantly larger
        than <varname>work_mem</varname>.  Larger settings might improve
        performance for vacuuming and for restoring database dumps.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-stack-depth" xreflabel="max_stack_depth">
      <term><varname>max_stack_depth</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_stack_depth</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the maximum safe depth of the server's execution stack.
        The ideal setting for this parameter is the actual stack size limit
        enforced by the kernel (as set by <literal>ulimit -s</> or local
        equivalent), less a safety margin of a megabyte or so.  The safety
        margin is needed because the stack depth is not checked in every
        routine in the server, but only in key potentially-recursive routines
        such as expression evaluation.  The default setting is two
        megabytes (<literal>2MB</>), which is conservatively small and
        unlikely to risk crashes.  However, it might be too small to allow
        execution of complex functions.  Only superusers can change this
        setting.
       </para>

       <para>
        Setting <varname>max_stack_depth</> higher than
        the actual kernel limit will mean that a runaway recursive function
        can crash an individual backend process.  On platforms where
        <productname>PostgreSQL</productname> can determine the kernel limit,
        it will not let you set this variable to an unsafe value.  However,
        not all platforms provide the information, so caution is recommended
        in selecting a value.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>
     <sect2 id="runtime-config-resource-fsm">
     <title>Free Space Map</title>

     <indexterm>
      <primary>free space map</primary>
     </indexterm>

     <para>
      These parameters control the size of the shared <firstterm>free space
      map</> (<acronym>FSM</>), which tracks the locations of unused space in the database.
      An undersized free space map can cause the database to consume
      increasing amounts of disk space over time, because free space that
      is not in the map cannot be re-used; instead <productname>PostgreSQL</>
      will request more disk space from the operating system when it needs
      to store new data.
      The last few lines displayed by a database-wide <command>VACUUM VERBOSE</>
      command can help in determining if the current settings are adequate.
      A <literal>NOTICE</> message is also printed during such an operation
      if the current settings are too low.
     </para>

     <para>
      Increasing these parameters might cause <productname>PostgreSQL</>
      to request more <systemitem class="osname">System V</> shared
      memory than your operating system's default configuration
      allows. See <xref linkend="sysvipc"> for information on how to
      adjust those parameters, if necessary.
     </para>

     <variablelist>
     <varlistentry id="guc-max-fsm-pages" xreflabel="max_fsm_pages">
      <term><varname>max_fsm_pages</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_fsm_pages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the maximum number of disk pages for which free space will
        be tracked in the shared free-space map.  Six bytes of shared memory
        are consumed for each page slot.  This setting must be at least
        16 * <varname>max_fsm_relations</varname>.  The default is chosen
        by <application>initdb</> depending on the amount of available memory,
        and can range from 20k to 200k pages.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-fsm-relations" xreflabel="max_fsm_relations">
      <term><varname>max_fsm_relations</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_fsm_relations</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the maximum number of relations (tables and indexes) for which
        free space will be tracked in the shared free-space map.  Roughly
        seventy bytes of shared memory are consumed for each slot.
        The default is one thousand relations.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

     <note>
      <para>
       See the <xref linkend="sql-vacuum" endterm="sql-vacuum-title">
       command for information on setting this parameter.
      </para>
     </note>

     </sect2>

     <sect2 id="runtime-config-resource-kernel">
     <title>Kernel Resource Usage</title>
     <variablelist>

     <varlistentry id="guc-max-files-per-process" xreflabel="max_files_per_process">
      <term><varname>max_files_per_process</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_files_per_process</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the maximum number of simultaneously open files allowed to each
        server subprocess. The default is one thousand files. If the kernel is enforcing
        a safe per-process limit, you don't need to worry about this setting.
        But on some platforms (notably, most BSD systems), the kernel will
        allow individual processes to open many more files than the system
        can really support when a large number of processes all try to open
        that many files. If you find yourself seeing <quote>Too many open
        files</> failures, try reducing this setting.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-shared-preload-libraries" xreflabel="shared_preload_libraries">
      <term><varname>shared_preload_libraries</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>shared_preload_libraries</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This variable specifies one or more shared libraries that are
        to be preloaded at server start. If more than one library is to be
        loaded, separate their names with commas. For example,
        <literal>'$libdir/mylib'</literal> would cause
        <literal>mylib.so</> (or on some platforms,
        <literal>mylib.sl</>) to be preloaded from the installation's
        standard library directory.
        This parameter can only be set at server start.
       </para>

       <para>
        <productname>PostgreSQL</productname> procedural language
        libraries can be preloaded in this way, typically by using the
        syntax <literal>'$libdir/plXXX'</literal> where
        <literal>XXX</literal> is <literal>pgsql</>, <literal>perl</>,
        <literal>tcl</>, or <literal>python</>.
       </para>

       <para>
        By preloading a shared library, the library startup time is avoided
        when the library is first used.  However, the time to start each new
        server process might increase slightly, even if that process never
        uses the library.  So this parameter is recommended only for
        libraries that will be used in most sessions. 
       </para>

     <note>
      <para>
        On Windows hosts, preloading a library at server start will not reduce
        the time required to start each new server process; each server process
        will re-load all preload libraries.  However, <varname>shared_preload_libraries
        </varname> is still useful on Windows hosts because some shared libraries may 
        need to perform certain operations that only take place at postmaster start
        (for example, a shared library may need to reserve lightweight locks
        or shared memory and you can't do that after the postmaster has started).
       </para>
      </note>
       <para>
        If a specified library is not found,
        the server will fail to start.
       </para>

       <para>
        Every  PostgreSQL-supported library has a <quote>magic
        block</> that is checked to guarantee compatibility.  
        For this reason, non-PostgreSQL libraries cannot be 
        loaded in this way.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

    <sect2 id="runtime-config-resource-vacuum-cost">
     <title id="runtime-config-resource-vacuum-cost-title">
       Cost-Based Vacuum Delay
     </title>

     <para>
      During the execution of <xref linkend="sql-vacuum"
      endterm="sql-vacuum-title"> and <xref linkend="sql-analyze"
      endterm="sql-analyze-title"> commands, the system maintains an
      internal counter that keeps track of the estimated cost of the
      various I/O operations that are performed.  When the accumulated
      cost reaches a limit (specified by
      <varname>vacuum_cost_limit</varname>), the process performing
      the operation will sleep for a while (specified by
      <varname>vacuum_cost_delay</varname>). Then it will reset the
      counter and continue execution.
     </para>

     <para>
      The intent of this feature is to allow administrators to reduce
      the I/O impact of these commands on concurrent database
      activity. There are many situations in which it is not very
      important that maintenance commands like
      <command>VACUUM</command> and <command>ANALYZE</command> finish
      quickly; however, it is usually very important that these
      commands do not significantly interfere with the ability of the
      system to perform other database operations. Cost-based vacuum
      delay provides a way for administrators to achieve this.
     </para>

     <para>
      This feature is disabled by default. To enable it, set the
      <varname>vacuum_cost_delay</varname> variable to a nonzero
      value.
     </para>

     <variablelist>
      <varlistentry id="guc-vacuum-cost-delay" xreflabel="vacuum_cost_delay">
       <term><varname>vacuum_cost_delay</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_delay</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The length of time, in milliseconds, that the process will sleep
         when the cost limit has been exceeded.
         The default value is zero, which disables the cost-based vacuum
         delay feature.  Positive values enable cost-based vacuuming.
         Note that on many systems, the effective resolution
         of sleep delays is 10 milliseconds; setting
         <varname>vacuum_cost_delay</varname> to a value that is
         not a multiple of 10 might have the same results as setting it
         to the next higher multiple of 10.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-page-hit" xreflabel="vacuum_cost_page_hit">
       <term><varname>vacuum_cost_page_hit</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_page_hit</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The estimated cost for vacuuming a buffer found in the shared buffer
         cache. It represents the cost to lock the buffer pool, lookup
         the shared hash table and scan the content of the page. The
         default value is one.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-page-miss" xreflabel="vacuum_cost_page_miss">
       <term><varname>vacuum_cost_page_miss</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_page_miss</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The estimated cost for vacuuming a buffer that has to be read from
         disk.  This represents the effort to lock the buffer pool,
         lookup the shared hash table, read the desired block in from
         the disk and scan its content. The default value is 10.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-page-dirty" xreflabel="vacuum_cost_page_dirty">
       <term><varname>vacuum_cost_page_dirty</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_page_dirty</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The estimated cost charged when vacuum modifies a block that was
         previously clean. It represents the extra I/O required to
         flush the dirty block out to disk again. The default value is
         20.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-vacuum-cost-limit" xreflabel="vacuum_cost_limit">
       <term><varname>vacuum_cost_limit</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>vacuum_cost_limit</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The accumulated cost that will cause the vacuuming process to sleep.
         The default value is 200.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <note>
      <para>
       There are certain operations that hold critical locks and should
       therefore complete as quickly as possible.  Cost-based vacuum
       delays do not occur during such operations.  Therefore it is
       possible that the cost accumulates far higher than the specified
       limit.  To avoid uselessly long delays in such cases, the actual
       delay is calculated as <varname>vacuum_cost_delay</varname> *
       <varname>accumulated_balance</varname> /
       <varname>vacuum_cost_limit</varname> with a maximum of
       <varname>vacuum_cost_delay</varname> * 4.
      </para>
     </note>
    </sect2>

    <sect2 id="runtime-config-resource-background-writer">
     <title>Background Writer</title>

     <para>
      There is a separate server
      process called the <firstterm>background writer</>, whose function
      is to issue writes of <quote>dirty</> shared buffers.  The intent is
      that server processes handling user queries should seldom or never have
      to wait for a write to occur, because the background writer will do it.
      However there is a net overall
      increase in I/O load, because a repeatedly-dirtied page might
      otherwise be written only once per checkpoint interval, but the
      background writer might write it several times in the same interval.
      The parameters discussed in this subsection can be used to
      tune the behavior for local needs.
     </para>

     <variablelist>
      <varlistentry id="guc-bgwriter-delay" xreflabel="bgwriter_delay">
       <term><varname>bgwriter_delay</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>bgwriter_delay</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         Specifies the delay between activity rounds for the
         background writer.  In each round the writer issues writes
         for some number of dirty buffers (controllable by the
         following parameters).  It then sleeps for <varname>bgwriter_delay</>
         milliseconds, and repeats.  The default value is 200 milliseconds
         (<literal>200ms</>). Note that on many systems, the effective
         resolution of sleep delays is 10 milliseconds; setting
         <varname>bgwriter_delay</> to a value that is not a multiple of
         10 might have the same results as setting it to the next higher
         multiple of 10.  This parameter can only be set in the
         <filename>postgresql.conf</> file or on the server command line.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-bgwriter-lru-maxpages" xreflabel="bgwriter_lru_maxpages">
       <term><varname>bgwriter_lru_maxpages</varname> (<type>integer</type>)</term>
       <indexterm>
        <primary><varname>bgwriter_lru_maxpages</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         In each round, no more than this many buffers will be written
         by the background writer.  Setting this to zero disables
         background writing (except for checkpoint activity).
         The default value is 100 buffers.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="guc-bgwriter-lru-multiplier" xreflabel="bgwriter_lru_multiplier">
       <term><varname>bgwriter_lru_multiplier</varname> (<type>floating point</type>)</term>
       <indexterm>
        <primary><varname>bgwriter_lru_multiplier</> configuration parameter</primary>
       </indexterm>
       <listitem>
        <para>
         The number of dirty buffers written in each round is based on the
         number of new buffers that have been needed by server processes
         during recent rounds.  The average recent need is multiplied by
         <varname>bgwriter_lru_multiplier</> to arrive at an estimate of the
         number of buffers that will be needed during the next round.  Dirty
         buffers are written until there are that many clean, reusable buffers
         available.  (However, no more than <varname>bgwriter_lru_maxpages</>
         buffers will be written per round.)
         Thus, a setting of 1.0 represents a <quote>just in time</> policy
         of writing exactly the number of buffers predicted to be needed.
         Larger values provide some cushion against spikes in demand,
         while smaller values intentionally leave writes to be done by
         server processes.
         The default is 2.0.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>

     <para>
      Smaller values of <varname>bgwriter_lru_maxpages</varname> and
      <varname>bgwriter_lru_multiplier</varname> reduce the extra I/O load
      caused by the background writer, but make it more likely that server
      processes will have to issue writes for themselves, delaying interactive
      queries.
     </para>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-wal">
    <title>Write Ahead Log</title>

   <para>
    See also <xref linkend="wal-configuration"> for details on WAL
    and checkpoint tuning.
   </para>

    <sect2 id="runtime-config-wal-settings">
     <title>Settings</title>
     <variablelist>
     
     <varlistentry id="guc-fsync" xreflabel="fsync">
      <indexterm>
       <primary><varname>fsync</> configuration parameter</primary>
      </indexterm>
      <term><varname>fsync</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        If this parameter is on, the <productname>PostgreSQL</> server
        will try to make sure that updates are physically written to
        disk, by issuing <function>fsync()</> system calls or various
        equivalent methods (see <xref linkend="guc-wal-sync-method">).
        This ensures that the database cluster can recover to a
        consistent state after an operating system or hardware crash.
       </para>

       <para>
        However, using <varname>fsync</varname> results in a
        performance penalty: when a transaction is committed,
        <productname>PostgreSQL</productname> must wait for the
        operating system to flush the write-ahead log to disk.  When
        <varname>fsync</varname> is disabled, the operating system is
        allowed to do its best in buffering, ordering, and delaying
        writes. This can result in significantly improved performance.
        However, if the system crashes, the results of the last few
        committed transactions might be lost in part or whole. In the
        worst case, unrecoverable data corruption might occur.
        (Crashes of the database software itself are <emphasis>not</>
        a risk factor here.  Only an operating-system-level crash
        creates a risk of corruption.)
       </para>

       <para>
        Due to the risks involved, there is no universally correct
        setting for <varname>fsync</varname>. Some administrators
        always disable <varname>fsync</varname>, while others only
        turn it off during initial bulk data loads, where there is a clear
        restart point if something goes wrong.  Others
        always leave <varname>fsync</varname> enabled. The default is
        to enable <varname>fsync</varname>, for maximum reliability.
        If you trust your operating system, your hardware, and your
        utility company (or your battery backup), you can consider
        disabling <varname>fsync</varname>.
       </para>

       <para>
        In many situations, turning off <xref linkend="guc-synchronous-commit">
        for noncritical transactions can provide much of the potential
        performance benefit of turning off <varname>fsync</varname>, without
        the attendant risks of data corruption.
       </para>

       <para>
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
        If you turn this parameter off, also consider turning off 
        <xref linkend="guc-full-page-writes">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-synchronous-commit" xreflabel="synchronous_commit">
      <term><varname>synchronous_commit</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>synchronous_commit</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies whether transaction commit will wait for WAL records
        to be written to disk before the command returns a <quote>success</>
        indication to the client.  The default, and safe, setting is
        <literal>on</>.  When <literal>off</>, there can be a delay between
        when success is reported to the client and when the transaction is
        really guaranteed to be safe against a server crash.  (The maximum
        delay is three times <xref linkend="guc-wal-writer-delay">.)  Unlike
        <xref linkend="guc-fsync">, setting this parameter to <literal>off</>
        does not create any risk of database inconsistency: a crash might
        result in some recent allegedly-committed transactions being lost, but
        the database state will be just the same as if those transactions had
        been aborted cleanly.  So, turning <varname>synchronous_commit</> off
        can be a useful alternative when performance is more important than
        exact certainty about the durability of a transaction.  For more
        discussion see <xref linkend="wal-async-commit">.
       </para>
       <para>
        This parameter can be changed at any time; the behavior for any
        one transaction is determined by the setting in effect when it
        commits.  It is therefore possible, and useful, to have some
        transactions commit synchronously and others asynchronously.
        For example, to make a single multi-statement transaction commit
        asynchronously when the default is the opposite, issue <command>SET
        LOCAL synchronous_commit TO OFF</> within the transaction.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-wal-sync-method" xreflabel="wal_sync_method">
      <term><varname>wal_sync_method</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>wal_sync_method</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Method used for forcing WAL updates out to disk.
        If <varname>fsync</varname> is off then this setting is irrelevant,
        since updates will not be forced out at all.
        Possible values are:
       </para>
       <itemizedlist>
        <listitem>
        <para>
         <literal>open_datasync</> (write WAL files with <function>open()</> option <symbol>O_DSYNC</>)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>fdatasync</> (call <function>fdatasync()</> at each commit)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>fsync_writethrough</> (call <function>fsync()</> at each commit, forcing write-through of any disk write cache)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>fsync</> (call <function>fsync()</> at each commit)
        </para>
        </listitem>
        <listitem>
        <para>
         <literal>open_sync</> (write WAL files with <function>open()</> option <symbol>O_SYNC</>)
        </para>
        </listitem>
       </itemizedlist>
       <para>
        Not all of these choices are available on all platforms.
        The default is the first method in the above list that is supported
        by the platform.
        The <literal>open_</>* options also use <literal>O_DIRECT</> if available.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-full-page-writes" xreflabel="full_page_writes">
      <indexterm>
       <primary><varname>full_page_writes</> configuration parameter</primary>
      </indexterm>
      <term><varname>full_page_writes</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        When this parameter is on, the <productname>PostgreSQL</> server
        writes the entire content of each disk page to WAL during the
        first modification of that page after a checkpoint.
        This is needed because
        a page write that is in process during an operating system crash might
        be only partially completed, leading to an on-disk page
        that contains a mix of old and new data.  The row-level change data
        normally stored in WAL will not be enough to completely restore
        such a page during post-crash recovery.  Storing the full page image
        guarantees that the page can be correctly restored, but at a price
        in increasing the amount of data that must be written to WAL.
        (Because WAL replay always starts from a checkpoint, it is sufficient
        to do this during the first change of each page after a checkpoint.
        Therefore, one way to reduce the cost of full-page writes is to
        increase the checkpoint interval parameters.)
       </para>

       <para>
        Turning this parameter off speeds normal operation, but
        might lead to a corrupt database after an operating system crash
        or power failure. The risks are similar to turning off
        <varname>fsync</>, though smaller.  It might be safe to turn off
        this parameter if you have hardware (such as a battery-backed disk
        controller) or file-system software that reduces
        the risk of partial page writes to an acceptably low level (e.g., ReiserFS 4).
       </para>

       <para>
        Turning off this parameter does not affect use of
        WAL archiving for point-in-time recovery (PITR)
        (see <xref linkend="continuous-archiving">).
       </para>

       <para>
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
        The default is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-buffers" xreflabel="wal_buffers">
      <term><varname>wal_buffers</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>wal_buffers</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The amount of memory used in shared memory for WAL data.  The
        default is 64 kilobytes (<literal>64kB</>).  The setting need only
        be large enough to hold the amount of WAL data generated by one
        typical transaction, since the data is written out to disk at
        every transaction commit.  This parameter can only be set at server
        start.
       </para>

       <para>
        Increasing this parameter might cause <productname>PostgreSQL</>
        to request more <systemitem class="osname">System V</> shared
        memory than your operating system's default configuration
        allows. See <xref linkend="sysvipc"> for information on how to
        adjust those parameters, if necessary.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-writer-delay" xreflabel="wal_writer_delay">
      <term><varname>wal_writer_delay</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>wal_writer_delay</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the delay between activity rounds for the WAL writer.
        In each round the writer will flush WAL to disk. It then sleeps for
        <varname>wal_writer_delay</> milliseconds, and repeats.  The default
        value is 200 milliseconds (<literal>200ms</>).  Note that on many
        systems, the effective resolution of sleep delays is 10 milliseconds;
        setting <varname>wal_writer_delay</> to a value that is not a multiple
        of 10 might have the same results as setting it to the next higher
        multiple of 10. This parameter can only be set in the
        <filename>postgresql.conf</> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-commit-delay" xreflabel="commit_delay">
      <term><varname>commit_delay</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>commit_delay</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Time delay between writing a commit record to the WAL buffer
        and flushing the buffer out to disk, in microseconds. A
        nonzero delay can allow multiple transactions to be committed
        with only one <function>fsync()</function> system call, if
        system load is high enough that additional transactions become
        ready to commit within the given interval. But the delay is
        just wasted if no other transactions become ready to
        commit. Therefore, the delay is only performed if at least
        <varname>commit_siblings</varname> other transactions are
        active at the instant that a server process has written its
        commit record. The default is zero (no delay).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-commit-siblings" xreflabel="commit_siblings">
      <term><varname>commit_siblings</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>commit_siblings</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Minimum number of concurrent open transactions to require
        before performing the <varname>commit_delay</> delay. A larger
        value makes it more probable that at least one other
        transaction will become ready to commit during the delay
        interval. The default is five transactions.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>
     <sect2 id="runtime-config-wal-checkpoints">
     <title>Checkpoints</title>

    <variablelist>
     <varlistentry id="guc-checkpoint-segments" xreflabel="checkpoint_segments">
      <term><varname>checkpoint_segments</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>checkpoint_segments</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Maximum number of log file segments between automatic WAL
        checkpoints (each segment is normally 16 megabytes). The default
        is three segments.  Increasing this parameter can increase the
        amount of time needed for crash recovery.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-checkpoint-timeout" xreflabel="checkpoint_timeout">
      <term><varname>checkpoint_timeout</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>checkpoint_timeout</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Maximum time between automatic WAL checkpoints, in
        seconds. The default is five minutes (<literal>5min</>).
        Increasing this parameter can increase the amount of time needed
        for crash recovery.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-checkpoint-completion-target" xreflabel="checkpoint_completion_target">
      <term><varname>checkpoint_completion_target</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>checkpoint_completion_target</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the target length of checkpoints, as a fraction of
        the checkpoint interval. The default is 0.5.

        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-checkpoint-warning" xreflabel="checkpoint_warning">
      <term><varname>checkpoint_warning</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>checkpoint_warning</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Write a message to the server log if checkpoints caused by
        the filling of checkpoint segment files happen closer together
        than this many seconds (which suggests that
        <varname>checkpoint_segments</> ought to be raised).  The default is
        30 seconds (<literal>30s</>).  Zero disables the warning.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
     </sect2>
     <sect2 id="runtime-config-wal-archiving">
     <title>Archiving</title>

    <variablelist>
     <varlistentry id="guc-archive-mode" xreflabel="archive_mode">
      <term><varname>archive_mode</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>archive_mode</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>archive_mode</> is enabled, completed WAL segments
        can be sent to archive storage by setting
        <xref linkend="guc-archive-command">.
        <varname>archive_mode</> and <varname>archive_command</> are
        separate variables so that <varname>archive_command</> can be
        changed without leaving archiving mode.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-archive-command" xreflabel="archive_command">
      <term><varname>archive_command</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>archive_command</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The shell command to execute to archive a completed segment of
        the WAL file series.  Any <literal>%p</> in the string is
        replaced by the path name of the file to archive, and any
        <literal>%f</> is replaced by the file name only.
        (The path name is relative to the working directory of the server,
        i.e., the cluster's data directory.)
        Use <literal>%%</> to embed an actual <literal>%</> character in the
        command. For more information see <xref
        linkend="backup-archiving-wal">.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.  It is ignored unless
        <varname>archive_mode</> was enabled at server start.
        If <varname>archive_command</> is an empty string (the default) while
        <varname>archive_mode</> is enabled, then WAL archiving is temporarily
        disabled, but the server continues to accumulate WAL segment files in
        the expectation that a command will soon be provided.
       </para>
       <para>
        It is important for the command to return a zero exit status if
        and only if it succeeds.  Examples:
<programlisting>
archive_command = 'cp "%p" /mnt/server/archivedir/"%f"'
archive_command = 'copy "%p" /mnt/server/archivedir/"%f"'  # Windows
</programlisting>
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-archive-timeout" xreflabel="archive_timeout">
      <term><varname>archive_timeout</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>archive_timeout</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The <xref linkend="guc-archive-command"> is only invoked on
        completed WAL segments. Hence, if your server generates little WAL
        traffic (or has slack periods where it does so), there could be a
        long delay between the completion of a transaction and its safe
        recording in archive storage.  To put a limit on how old unarchived
        data can be, you can set <varname>archive_timeout</> to force the
        server to switch to a new WAL segment file periodically.  When this
        parameter is greater than zero, the server will switch to a new
        segment file whenever this many seconds have elapsed since the last
        segment file switch.  Note that archived files that are closed early
        due to a forced switch are still the same length as completely full
        files.  Therefore, it is unwise to use a very short
        <varname>archive_timeout</> &mdash; it will bloat your archive
        storage.  <varname>archive_timeout</> settings of a minute or so are
        usually reasonable.  This parameter can only be set in the
        <filename>postgresql.conf</> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-query">
    <title>Query Planning</title>

    <sect2 id="runtime-config-query-enable">
     <title>Planner Method Configuration</title>

      <para>
       These configuration parameters provide a crude method of
       influencing the query plans chosen by the query optimizer. If
       the default plan chosen by the optimizer for a particular query
       is not optimal, a temporary solution can be found by using one
       of these configuration parameters to force the optimizer to
       choose a different plan.  Turning one of these settings off
       permanently is seldom a good idea, however.
       Better ways to improve the quality of the
       plans chosen by the optimizer include adjusting the <xref
       linkend="runtime-config-query-constants"
       endterm="runtime-config-query-constants-title">, running <xref
       linkend="sql-analyze" endterm="sql-analyze-title"> more
       frequently, increasing the value of the <xref
       linkend="guc-default-statistics-target"> configuration parameter,
       and increasing the amount of statistics collected for
       specific columns using <command>ALTER TABLE SET
       STATISTICS</command>.
      </para>

     <variablelist>
     <varlistentry id="guc-enable-bitmapscan" xreflabel="enable_bitmapscan">
      <term><varname>enable_bitmapscan</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary>bitmap scan</primary>
      </indexterm>
      <indexterm>
       <primary><varname>enable_bitmapscan</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of bitmap-scan plan
        types. The default is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-hashagg" xreflabel="enable_hashagg">
      <term><varname>enable_hashagg</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_hashagg</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of hashed
        aggregation plan types. The default is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-hashjoin" xreflabel="enable_hashjoin">
      <term><varname>enable_hashjoin</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_hashjoin</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of hash-join plan
        types. The default is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-indexscan" xreflabel="enable_indexscan">
      <term><varname>enable_indexscan</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary>index scan</primary>
      </indexterm>
      <indexterm>
       <primary><varname>enable_indexscan</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of index-scan plan
        types. The default is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-mergejoin" xreflabel="enable_mergejoin">
      <term><varname>enable_mergejoin</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_mergejoin</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of merge-join plan
        types. The default is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-nestloop" xreflabel="enable_nestloop">
      <term><varname>enable_nestloop</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_nestloop</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of nested-loop join
        plans. It's not possible to suppress nested-loop joins entirely,
        but turning this variable off discourages the planner from using
        one if there are other methods available. The default is
        <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-seqscan" xreflabel="enable_seqscan">
      <term><varname>enable_seqscan</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary>sequential scan</primary>
      </indexterm>
      <indexterm>
       <primary><varname>enable_seqscan</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of sequential scan
        plan types. It's not possible to suppress sequential scans
        entirely, but turning this variable off discourages the planner
        from using one if there are other methods available. The
        default is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-sort" xreflabel="enable_sort">
      <term><varname>enable_sort</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_sort</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of explicit sort
        steps. It's not possible to suppress explicit sorts entirely,
        but turning this variable off discourages the planner from
        using one if there are other methods available. The default
        is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-enable-tidscan" xreflabel="enable_tidscan">
      <term><varname>enable_tidscan</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>enable_tidscan</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of <acronym>TID</>
        scan plan types. The default is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
     </sect2>
     <sect2 id="runtime-config-query-constants">
     <title id="runtime-config-query-constants-title">
      Planner Cost Constants
     </title>

    <para>
     The <firstterm>cost</> variables described in this section are measured
     on an arbitrary scale.  Only their relative values matter, hence
     scaling them all up or down by the same factor will result in no change
     in the planner's choices.  Traditionally, these variables have been
     referenced to sequential page fetches as the unit of cost; that is,
     <varname>seq_page_cost</> is conventionally set to <literal>1.0</>
     and the other cost variables are set with reference to that.  But
     you can use a different scale if you prefer, such as actual execution
     times in milliseconds on a particular machine.
    </para>

   <note>
    <para>
     Unfortunately, there is no well-defined method for determining ideal
     values for the cost variables.  They are best treated as averages over
     the entire mix of queries that a particular installation will get.  This
     means that changing them on the basis of just a few experiments is very
     risky.
    </para>
   </note>

     <variablelist>

     <varlistentry id="guc-seq-page-cost" xreflabel="seq_page_cost">
      <term><varname>seq_page_cost</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>seq_page_cost</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of a disk page fetch
        that is part of a series of sequential fetches.  The default is 1.0.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-random-page-cost" xreflabel="random_page_cost">
      <term><varname>random_page_cost</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>random_page_cost</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of a
        non-sequentially-fetched disk page.  The default is 4.0.
        Reducing this value relative to <varname>seq_page_cost</>
        will cause the system to prefer index scans; raising it will
        make index scans look relatively more expensive.  You can raise
        or lower both values together to change the importance of disk I/O
        costs relative to CPU costs, which are described by the following
        parameters.
       </para>

       <tip>
        <para>
         Although the system will let you set <varname>random_page_cost</> to
         less than <varname>seq_page_cost</>, it is not physically sensible
         to do so.  However, setting them equal makes sense if the database
         is entirely cached in RAM, since in that case there is no penalty
         for touching pages out of sequence.  Also, in a heavily-cached
         database you should lower both values relative to the CPU parameters,
         since the cost of fetching a page already in RAM is much smaller
         than it would normally be.
        </para>
       </tip>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-cpu-tuple-cost" xreflabel="cpu_tuple_cost">
      <term><varname>cpu_tuple_cost</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>cpu_tuple_cost</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing
        each row during a query.
        The default is 0.01.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-cpu-index-tuple-cost" xreflabel="cpu_index_tuple_cost">
      <term><varname>cpu_index_tuple_cost</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>cpu_index_tuple_cost</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing
        each index entry during an index scan.
        The default is 0.005.
       </para>
      </listitem>
     </varlistentry>
    
     <varlistentry id="guc-cpu-operator-cost" xreflabel="cpu_operator_cost">
      <term><varname>cpu_operator_cost</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>cpu_operator_cost</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the cost of processing each
        operator or function executed during a query.
        The default is 0.0025.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-effective-cache-size" xreflabel="effective_cache_size">
      <term><varname>effective_cache_size</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>effective_cache_size</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's assumption about the effective size of the
        disk cache that is available to a single query.  This is
        factored into estimates of the cost of using an index; a
        higher value makes it more likely index scans will be used, a
        lower value makes it more likely sequential scans will be
        used. When setting this parameter you should consider both
        <productname>PostgreSQL</productname>'s shared buffers and the
        portion of the kernel's disk cache that will be used for
        <productname>PostgreSQL</productname> data files.  Also, take
        into account the expected number of concurrent queries on different
        tables, since they will have to share the available
        space.  This parameter has no effect on the size of shared
        memory allocated by <productname>PostgreSQL</productname>, nor
        does it reserve kernel disk cache; it is used only for estimation
        purposes.  The default is 128 megabytes (<literal>128MB</>).
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect2>
     <sect2 id="runtime-config-query-geqo">
     <title>Genetic Query Optimizer</title>

     <variablelist>

     <varlistentry id="guc-geqo" xreflabel="geqo">
      <indexterm>
       <primary>genetic query optimization</primary>
      </indexterm>
      <indexterm>
       <primary>GEQO</primary>
       <see>genetic query optimization</see>
      </indexterm>
      <indexterm>
       <primary><varname>geqo</> configuration parameter</primary>
      </indexterm>
      <term><varname>geqo</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Enables or disables genetic query optimization, which is an
        algorithm that attempts to do query planning without
        exhaustive searching. This is on by default. The
        <varname>geqo_threshold</varname> variable provides a more
        granular way to disable GEQO for certain classes of queries.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-threshold" xreflabel="geqo_threshold">
      <term><varname>geqo_threshold</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>geqo_threshold</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Use genetic query optimization to plan queries with at least
        this many <literal>FROM</> items involved. (Note that a
        <literal>FULL OUTER JOIN</> construct counts as only one <literal>FROM</>
        item.) The default is 12. For simpler queries it is usually best
        to use the deterministic, exhaustive planner, but for queries with
        many tables the deterministic planner takes too long.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-effort" xreflabel="geqo_effort">
      <term><varname>geqo_effort</varname>
      (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>geqo_effort</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the trade off between planning time and query plan
        efficiency in GEQO. This variable must be an integer in the
        range from 1 to 10. The default value is five. Larger values
        increase the time spent doing query planning, but also
        increase the likelihood that an efficient query plan will be
        chosen.
       </para>

       <para>
        <varname>geqo_effort</varname> doesn't actually do anything
        directly; it is only used to compute the default values for
        the other variables that influence GEQO behavior (described
        below). If you prefer, you can set the other parameters by
        hand instead.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-pool-size" xreflabel="geqo_pool_size">
      <term><varname>geqo_pool_size</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>geqo_pool_size</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the pool size used by GEQO. The pool size is the
        number of individuals in the genetic population.  It must be
        at least two, and useful values are typically 100 to 1000.  If
        it is set to zero (the default setting) then a suitable
        default is chosen based on <varname>geqo_effort</varname> and
        the number of tables in the query.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-generations" xreflabel="geqo_generations">
      <term><varname>geqo_generations</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>geqo_generations</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the number of generations used by GEQO.  Generations
        specifies the number of iterations of the algorithm.  It must
        be at least one, and useful values are in the same range as
        the pool size.  If it is set to zero (the default setting)
        then a suitable default is chosen based on
        <varname>geqo_pool_size</varname>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-geqo-selection-bias" xreflabel="geqo_selection_bias">
      <term><varname>geqo_selection_bias</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>geqo_selection_bias</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the selection bias used by GEQO. The selection bias
        is the selective pressure within the population. Values can be
        from 1.50 to 2.00; the latter is the default.
       </para>
      </listitem>
     </varlistentry>
     
     </variablelist>
    </sect2>
     <sect2 id="runtime-config-query-other">
     <title>Other Planner Options</title>

     <variablelist>

     <varlistentry id="guc-default-statistics-target" xreflabel="default_statistics_target">
      <term><varname>default_statistics_target</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>default_statistics_target</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the default statistics target for table columns that have
        not had a column-specific target set via <command>ALTER TABLE
        SET STATISTICS</>.  Larger values increase the time needed to
        do <command>ANALYZE</>, but might improve the quality of the
        planner's estimates. The default is 10. For more information
        on the use of statistics by the <productname>PostgreSQL</>
        query planner, refer to <xref linkend="planner-stats">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-constraint-exclusion" xreflabel="constraint_exclusion">
      <term><varname>constraint_exclusion</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary>constraint exclusion</primary>
      </indexterm>
      <indexterm>
       <primary><varname>constraint_exclusion</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables or disables the query planner's use of table constraints to
        optimize queries.  The default is <literal>off</>.
       </para>

       <para>
        When this parameter is <literal>on</>, the planner compares
        query conditions with table <literal>CHECK</> constraints, and
        omits scanning tables for which the conditions contradict the
        constraints.  For example:

<programlisting>
CREATE TABLE parent(key integer, ...);
CREATE TABLE child1000(check (key between 1000 and 1999)) INHERITS(parent);
CREATE TABLE child2000(check (key between 2000 and 2999)) INHERITS(parent);
...
SELECT * FROM parent WHERE key = 2400;
</programlisting>

        With constraint exclusion enabled, this <command>SELECT</>
        will not scan <structname>child1000</> at all.  This can
        improve performance when inheritance is used to build
        partitioned tables.
       </para>

       <para>
        Currently, <varname>constraint_exclusion</> is disabled by
        default because the constraint checks are relatively
        expensive, and in many circumstances will yield no savings.
        It is recommended to turn this on only if you are actually
        using partitioned tables designed to take advantage of the
        feature.
       </para>

       <para>
        Refer to <xref linkend="ddl-partitioning"> for more information
        on using constraint exclusion and partitioning.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-cursor-tuple-fraction" xreflabel="cursor_tuple_fraction">
      <term><varname>cursor_tuple_fraction</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>cursor_tuple_fraction</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the planner's estimate of the fraction of a cursor's rows that
        will be retrieved.  The default is 0.1.  Smaller values of this
        setting bias the planner towards using <quote>fast start</> plans
        for cursors, which will retrieve the first few rows quickly while
        perhaps taking a long time to fetch all rows.  Larger values
        put more emphasis on the total estimated time.  At the maximum
        setting of 1.0, cursors are planned exactly like regular queries,
        considering only the total estimated time and not how soon the
        first rows might be delivered.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-from-collapse-limit" xreflabel="from_collapse_limit">
      <term><varname>from_collapse_limit</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>from_collapse_limit</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The planner will merge sub-queries into upper queries if the
        resulting <literal>FROM</literal> list would have no more than
        this many items.  Smaller values reduce planning time but might
        yield inferior query plans.  The default is eight.  It is usually
        wise to keep this less than <xref linkend="guc-geqo-threshold">.
        For more information see <xref linkend="explicit-joins">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-join-collapse-limit" xreflabel="join_collapse_limit">
      <term><varname>join_collapse_limit</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>join_collapse_limit</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The planner will rewrite explicit <literal>JOIN</>
        constructs (except <literal>FULL JOIN</>s) into lists of
        <literal>FROM</> items whenever a list of no more than this many items
        would result.  Smaller values reduce planning time but might
        yield inferior query plans.
       </para>

       <para>
        By default, this variable is set the same as
        <varname>from_collapse_limit</varname>, which is appropriate
        for most uses. Setting it to 1 prevents any reordering of
        explicit <literal>JOIN</>s. Thus, the explicit join order
        specified in the query will be the actual order in which the
        relations are joined. The query planner does not always choose
        the optimal join order; advanced users can elect to
        temporarily set this variable to 1, and then specify the join
        order they desire explicitly.
        For more information see <xref linkend="explicit-joins">.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-logging">
    <title>Error Reporting and Logging</title>

    <indexterm zone="runtime-config-logging">
     <primary>server log</primary>
    </indexterm>

    <sect2 id="runtime-config-logging-where">
     <title>Where To Log</title>

     <indexterm zone="runtime-config-logging-where">
      <primary>where to log</primary>
     </indexterm>

     <variablelist>

     <varlistentry id="guc-log-destination" xreflabel="log_destination">
      <term><varname>log_destination</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_destination</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        <productname>PostgreSQL</productname> supports several methods
         for logging server messages, including
         <systemitem>stderr</systemitem>, <systemitem>csvlog</systemitem> and
         <systemitem>syslog</systemitem>. On Windows, 
         <systemitem>eventlog</systemitem> is also supported. Set this
         parameter to a list of desired log destinations separated by
         commas. The default is to log to <systemitem>stderr</systemitem> 
         only.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
       </para>
       <para>
        If <systemitem>csvlog</> is included in <varname>log_destination</>,
        log entries are output in <quote>comma separated
        value</> format, which is convenient for loading them into programs.
        See <xref linkend="runtime-config-logging-csvlog"> for details.
        <varname>logging_collector</varname> must be enabled to generate 
        CSV-format log output.
       </para>

       <note>
        <para>
         On most Unix systems, you will need to alter the configuration of
         your system's <application>syslog</application> daemon in order
         to make use of the <systemitem>syslog</systemitem> option for
         <varname>log_destination</>.  <productname>PostgreSQL</productname>
         can log to <application>syslog</application> facilities
         <literal>LOCAL0</> through <literal>LOCAL7</> (see <xref
         linkend="guc-syslog-facility">), but the default
         <application>syslog</application> configuration on most platforms
         will discard all such messages.  You will need to add something like
<programlisting>
local0.*    /var/log/postgresql
</programlisting>
         to the  <application>syslog</application> daemon's configuration file
         to make it work.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-logging-collector" xreflabel="logging_collector">
      <term><varname>logging_collector</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>logging_collector</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         This parameter allows messages sent to <application>stderr</>,
         and CSV-format log output, to be
         captured and redirected into log files.
         This approach is often more useful than
         logging to <application>syslog</>, since some types of messages
         might not appear in <application>syslog</> output (a common example
         is dynamic-linker failure messages).
         This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-directory" xreflabel="log_directory">
      <term><varname>log_directory</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_directory</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>logging_collector</> is enabled, 
        this parameter determines the directory in which log files will be created.
        It can be specified as an absolute path, or relative to the
        cluster data directory.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-filename" xreflabel="log_filename">
      <term><varname>log_filename</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_filename</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>logging_collector</varname> is enabled,
        this parameter sets the file names of the created log files.  The value
        is treated as a <systemitem>strftime</systemitem> pattern,
        so <literal>%</literal>-escapes can be used to specify time-varying
        file names.  (Note that if there are
        any time-zone-dependent <literal>%</literal>-escapes, the computation
        is done in the zone specified by <xref linkend="guc-log-timezone">.)
        If no <literal>%</literal>-escapes are present,
        <productname>PostgreSQL</productname> will append the epoch of the new
        log file's creation time.  For example, if
        <varname>log_filename</varname> were <literal>server_log</literal>,
        then the chosen file name would be <literal>server_log.1093827753</>
        for a log starting at Sun Aug 29 19:02:33 2004 MST.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
       <para>
        If CSV-format output is enabled in <varname>log_destination</>,
        <literal>.csv</> will be appended to the timestamped 
        log file name to create the file name for CSV-format output.
        (If <varname>log_filename</> ends in <literal>.log</>, the suffix is
        replaced instead.)
        In the case of the example above, the CSV
        file name will be <literal>server_log.1093827753.csv</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-rotation-age" xreflabel="log_rotation_age">
      <term><varname>log_rotation_age</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>log_rotation_age</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>logging_collector</varname> is enabled,
        this parameter determines the maximum lifetime of an individual log file.
        After this many minutes have elapsed, a new log file will
        be created.  Set to zero to disable time-based creation of
        new log files.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-rotation-size" xreflabel="log_rotation_size">
      <term><varname>log_rotation_size</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>log_rotation_size</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>logging_collector</varname> is enabled,
        this parameter determines the maximum size of an individual log file.
        After this many kilobytes have been emitted into a log file,
        a new log file will be created.  Set to zero to disable size-based
        creation of new log files.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-truncate-on-rotation" xreflabel="log_truncate_on_rotation">
      <term><varname>log_truncate_on_rotation</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_truncate_on_rotation</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When <varname>logging_collector</varname> is enabled,
        this parameter will cause <productname>PostgreSQL</productname> to truncate (overwrite),
        rather than append to, any existing log file of the same name.
        However, truncation will occur only when a new file is being opened
        due to time-based rotation, not during server startup or size-based
        rotation.  When off, pre-existing files will be appended to in
        all cases.  For example, using this setting in combination with
        a <varname>log_filename</varname> like <literal>postgresql-%H.log</literal>
        would result in generating twenty-four hourly log files and then
        cyclically overwriting them.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
       <para>
        Example:  To keep 7 days of logs, one log file per day named
        <literal>server_log.Mon</literal>, <literal>server_log.Tue</literal>, 
        etc, and automatically overwrite last week's log with this week's log,
        set <varname>log_filename</varname> to <literal>server_log.%a</literal>, 
        <varname>log_truncate_on_rotation</varname> to <literal>on</literal>, and 
        <varname>log_rotation_age</varname> to <literal>1440</literal>.
       </para>
       <para>
        Example: To keep 24 hours of logs, one log file per hour, but 
        also rotate sooner if the log file size exceeds 1GB, set 
        <varname>log_filename</varname> to <literal>server_log.%H%M</literal>, 
        <varname>log_truncate_on_rotation</varname> to <literal>on</literal>, 
        <varname>log_rotation_age</varname> to <literal>60</literal>, and 
        <varname>log_rotation_size</varname> to <literal>1000000</literal>.
        Including <literal>%M</> in <varname>log_filename</varname> allows
        any size-driven rotations that might occur to select a file name
        different from the hour's initial file name.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-syslog-facility" xreflabel="syslog_facility">
      <term><varname>syslog_facility</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>syslog_facility</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When logging to <application>syslog</> is enabled, this parameter
        determines the <application>syslog</application>
        <quote>facility</quote> to be used.  You can choose
        from <literal>LOCAL0</>, <literal>LOCAL1</>,
        <literal>LOCAL2</>, <literal>LOCAL3</>, <literal>LOCAL4</>,
        <literal>LOCAL5</>, <literal>LOCAL6</>, <literal>LOCAL7</>;
        the default is <literal>LOCAL0</>. See also the
        documentation of your system's
        <application>syslog</application> daemon.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-syslog-ident" xreflabel="syslog_ident">
      <term><varname>syslog_ident</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>syslog_identity</> configuration parameter</primary>
      </indexterm>
       <listitem>
        <para>
         When logging to <application>syslog</> is enabled, this parameter
         determines the program name used to identify
         <productname>PostgreSQL</productname> messages in
         <application>syslog</application> logs. The default is
         <literal>postgres</literal>.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line.
        </para>
       </listitem>
      </varlistentry>
      
      </variablelist>
    </sect2>
     <sect2 id="runtime-config-logging-when">
     <title>When To Log</title>

     <variablelist>

     <varlistentry id="guc-client-min-messages" xreflabel="client_min_messages">
      <term><varname>client_min_messages</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>client_min_messages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls which message levels are sent to the client.
        Valid values are <literal>DEBUG5</>,
        <literal>DEBUG4</>, <literal>DEBUG3</>, <literal>DEBUG2</>,
        <literal>DEBUG1</>, <literal>LOG</>, <literal>NOTICE</>,
        <literal>WARNING</>, <literal>ERROR</>, <literal>FATAL</>,
        and <literal>PANIC</>.  Each level
        includes all the levels that follow it.  The later the level,
        the fewer messages are sent.  The default is
        <literal>NOTICE</>.  Note that <literal>LOG</> has a different
        rank here than in <varname>log_min_messages</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-min-messages" xreflabel="log_min_messages">
      <term><varname>log_min_messages</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_min_messages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls which message levels are written to the server log.
        Valid values are <literal>DEBUG5</>, <literal>DEBUG4</>,
        <literal>DEBUG3</>, <literal>DEBUG2</>, <literal>DEBUG1</>,
        <literal>INFO</>, <literal>NOTICE</>, <literal>WARNING</>,
        <literal>ERROR</>, <literal>LOG</>, <literal>FATAL</>, and
        <literal>PANIC</>.  Each level includes all the levels that
        follow it.  The later the level, the fewer messages are sent
        to the log.  The default is <literal>WARNING</>.  Note that
        <literal>LOG</> has a different rank here than in
        <varname>client_min_messages</>.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-error-verbosity" xreflabel="log_error_verbosity">
      <term><varname>log_error_verbosity</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_error_verbosity</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls the amount of detail written in the server log for each
        message that is logged.  Valid values are <literal>TERSE</>,
        <literal>DEFAULT</>, and <literal>VERBOSE</>, each adding more
        fields to displayed messages.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-min-error-statement" xreflabel="log_min_error_statement">
      <term><varname>log_min_error_statement</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_min_error_statement</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls whether or not the SQL statement that causes an error
        condition will be recorded in the server log.  The current
        SQL statement is included in the log entry for any message of
        the specified severity or higher.
        Valid values are <literal>DEBUG5</literal>,
        <literal>DEBUG4</literal>, <literal>DEBUG3</literal>,
        <literal>DEBUG2</literal>, <literal>DEBUG1</literal>,
        <literal>INFO</literal>, <literal>NOTICE</literal>,
        <literal>WARNING</literal>, <literal>ERROR</literal>,
        <literal>LOG</literal>,
        <literal>FATAL</literal>, and <literal>PANIC</literal>.
        The default is <literal>ERROR</literal>, which means statements
        causing errors, log messages, fatal errors, or panics will be logged.
        To effectively turn off logging of failing statements,
        set this parameter to <literal>PANIC</literal>.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-log-min-duration-statement" xreflabel="log_min_duration_statement">
      <term><varname>log_min_duration_statement</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>log_min_duration_statement</> configuration parameter</primary>
      </indexterm>
       <listitem>
        <para>
         Causes the duration of each completed statement to be logged
         if the statement ran for at least the specified number of
         milliseconds.  Setting this to zero prints all statement durations.
         Minus-one (the default) disables logging statement durations.
         For example, if you set it to <literal>250ms</literal>
         then all SQL statements that run 250ms or longer will be
         logged.  Enabling this parameter can be helpful in tracking down
         unoptimized queries in your applications.
         Only superusers can change this setting.
        </para>

        <para>
         For clients using extended query protocol, durations of the Parse,
         Bind, and Execute steps are logged independently.
        </para>

       <note>
        <para>
         When using this option together with
         <xref linkend="guc-log-statement">,
         the text of statements that are logged because of
         <varname>log_statement</> will not be repeated in the
         duration log message.
         If you are not using <application>syslog</>, it is recommended 
         that you log the PID or session ID using
         <xref linkend="guc-log-line-prefix">
         so that you can link the statement message to the later
         duration message using the process ID or session ID.
        </para>
       </note>
       </listitem>
      </varlistentry>

     <varlistentry id="guc-silent-mode" xreflabel="silent_mode">
      <term><varname>silent_mode</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>silent_mode</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Runs the server silently. If this parameter is set, the server
        will automatically run in background and any controlling
        terminals are disassociated.
        The server's standard output and standard error are redirected
        to <literal>/dev/null</>, so any messages sent to them will be lost.
        Unless <application>syslog</> logging is selected or
        <varname>logging_collector</> is enabled, using this parameter
        is discouraged because it makes it impossible to see error messages.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    <para>
     <xref linkend="runtime-config-severity-levels"> explains the message
     severity levels used by <productname>PostgreSQL</>.  If logging output
     is sent to <systemitem>syslog</systemitem> or Windows'
     <systemitem>eventlog</systemitem>, the severity levels are translated
     as shown in the table.
    </para>

    <table id="runtime-config-severity-levels">
     <title>Message severity levels</title>
     <tgroup cols="4">
      <thead>
       <row>
        <entry>Severity</entry>
        <entry>Usage</entry>
        <entry><systemitem>syslog</></entry>
        <entry><systemitem>eventlog</></entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><literal>DEBUG1..DEBUG5</></entry>
        <entry>Provides successively-more-detailed information for use by
         developers.</entry>
        <entry><literal>DEBUG</></entry>
        <entry><literal>INFORMATION</></entry>
       </row>

       <row>
        <entry><literal>INFO</></entry>
        <entry>Provides information implicitly requested by the user,
         e.g., output from <command>VACUUM VERBOSE</>.</entry>
        <entry><literal>INFO</></entry>
        <entry><literal>INFORMATION</></entry>
       </row>

       <row>
        <entry><literal>NOTICE</></entry>
        <entry>Provides information that might be helpful to users, e.g.,
         notice of truncation of long identifiers.</entry>
        <entry><literal>NOTICE</></entry>
        <entry><literal>INFORMATION</></entry>
       </row>

       <row>
        <entry><literal>WARNING</></entry>
        <entry>Provides warnings of likely problems, e.g., <command>COMMIT</>
         outside a transaction block.</entry>
        <entry><literal>NOTICE</></entry>
        <entry><literal>WARNING</></entry>
       </row>

       <row>
        <entry><literal>ERROR</></entry>
        <entry>Reports an error that caused the current command to
         abort.</entry>
        <entry><literal>WARNING</></entry>
        <entry><literal>ERROR</></entry>
       </row>

       <row>
        <entry><literal>LOG</></entry>
        <entry>Reports information of interest to administrators, e.g.,
         checkpoint activity.</entry>
        <entry><literal>INFO</></entry>
        <entry><literal>INFORMATION</></entry>
       </row>

       <row>
        <entry><literal>FATAL</></entry>
        <entry>Reports an error that caused the current session to
         abort.</entry>
        <entry><literal>ERR</></entry>
        <entry><literal>ERROR</></entry>
       </row>

       <row>
        <entry><literal>PANIC</></entry>
        <entry>Reports an error that caused all database sessions to abort.</entry>
        <entry><literal>CRIT</></entry>
        <entry><literal>ERROR</></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    </sect2>
     <sect2 id="runtime-config-logging-what">
     <title>What To Log</title>

     <variablelist>

     <varlistentry>
      <term><varname>debug_print_parse</varname> (<type>boolean</type>)</term>
      <term><varname>debug_print_rewritten</varname> (<type>boolean</type>)</term>
      <term><varname>debug_print_plan</varname> (<type>boolean</type>)</term>
      <term><varname>debug_pretty_print</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>debug_print_parse</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>debug_print_rewritten</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>debug_print_plan</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>debug_pretty_print</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        These parameters enable various debugging output to be emitted.
        For each executed query, they print
        the resulting parse tree, the query rewriter output, or the
        execution plan.  <varname>debug_pretty_print</varname> indents
        these displays to produce a more readable but much longer
        output format.  <varname>client_min_messages</varname> or
        <varname>log_min_messages</varname> must be
        <literal>DEBUG1</literal> or lower to actually send this output
        to the client or the server log, respectively.
        These parameters are off by default.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-checkpoints" xreflabel="log_checkpoints">
      <term><varname>log_checkpoints</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_checkpoints</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Causes checkpoints to be logged in the server log. Some
        statistics about each checkpoint are included in the log messages,
        including the number of buffers written and the time spent writing
        them.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line. The default is off.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-connections" xreflabel="log_connections">
      <term><varname>log_connections</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_connections</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Causes each attempted connection to the server to be logged,
        as well as successful completion of client authentication.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.  The default is off.
       </para>

       <note>
        <para>
         Some client programs, like <application>psql</>, attempt 
         to connect twice while determining if a password is required, so 
         duplicate <quote>connection received</> messages do not
         necessarily indicate a problem.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-disconnections" xreflabel="log_disconnections">
      <term><varname>log_disconnections</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_disconnections</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This outputs a line in the server log similar to
        <varname>log_connections</varname> but at session termination,
        and includes the duration of the session.  This is off by
        default.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>


     <varlistentry id="guc-log-duration" xreflabel="log_duration">
      <term><varname>log_duration</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_duration</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Causes the duration of every completed statement to be logged.
        The default is <literal>off</>.
        Only superusers can change this setting.
       </para>

       <para>
        For clients using extended query protocol, durations of the Parse,
        Bind, and Execute steps are logged independently.
       </para>

       <note>
        <para>
         The difference between setting this option and setting
         <xref linkend="guc-log-min-duration-statement"> to zero is that
         exceeding <varname>log_min_duration_statement</> forces the text of
         the query to be logged, but this option doesn't.  Thus, if
         <varname>log_duration</> is <literal>on</> and
         <varname>log_min_duration_statement</> has a positive value, all
         durations are logged but the query text is included only for
         statements exceeding the threshold.  This behavior can be useful for
         gathering statistics in high-load installations.
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-hostname" xreflabel="log_hostname">
      <term><varname>log_hostname</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_hostname</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        By default, connection log messages only show the IP address of the
        connecting host. Turning on this parameter causes logging of the
        host name as well.  Note that depending on your host name resolution
        setup this might impose a non-negligible performance penalty.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry id="guc-log-line-prefix" xreflabel="log_line_prefix">
      <term><varname>log_line_prefix</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_line_prefix</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
         This is a <function>printf</>-style string that is output at the
         beginning of each log line.
         <literal>%</> characters begin <quote>escape sequences</>
         that are replaced with status information as outlined below.
         Unrecognized escapes are ignored. Other
         characters are copied straight to the log line. Some escapes are
         only recognized by session processes, and do not apply to
         background processes such as the main server process.
         This parameter can only be set in the <filename>postgresql.conf</>
         file or on the server command line. The default is an empty string.

         <informaltable>
          <tgroup cols="3">
           <thead>
            <row>
             <entry>Escape</entry>
             <entry>Effect</entry>
             <entry>Session only</entry>
             </row>
            </thead>
           <tbody>
            <row>
             <entry><literal>%u</literal></entry>
             <entry>User name</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%d</literal></entry>
             <entry>Database name</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%r</literal></entry>
             <entry>Remote host name or IP address, and remote port</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%h</literal></entry>
             <entry>Remote host name or IP address</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%p</literal></entry>
             <entry>Process ID</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%t</literal></entry>
             <entry>Time stamp without milliseconds</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%m</literal></entry>
             <entry>Time stamp with milliseconds</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%i</literal></entry>
             <entry>Command tag: type of session's current command</entry>
             <entry>yes</entry>
            </row>
            <row>
             <entry><literal>%c</literal></entry>
             <entry>Session ID: see below</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%l</literal></entry>
             <entry>Number of the log line for each session or process, starting at 1</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%s</literal></entry>
             <entry>Process start time stamp</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%v</literal></entry>
             <entry>Virtual transaction ID (backendID/localXID)</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%x</literal></entry>
             <entry>Transaction ID (0 if none is assigned)</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%q</literal></entry>
             <entry>Produces no output, but tells non-session
             processes to stop at this point in the string; ignored by
             session processes</entry>
             <entry>no</entry>
            </row>
            <row>
             <entry><literal>%%</literal></entry>
             <entry>Literal <literal>%</></entry>
             <entry>no</entry>
            </row>
           </tbody>
          </tgroup>
         </informaltable>

         The <literal>%c</> escape prints a quasi-unique session identifier,
         consisting of two 4-byte hexadecimal numbers (without leading zeros)
         separated by a dot.  The numbers are the process start time and the
         process ID, so <literal>%c</> can also be used as a space saving way
         of printing those items.
       </para>

       <tip>
        <para>
         If you set a nonempty value for <varname>log_line_prefix</>,
         you should usually make its last character be a space, to provide
         visual separation from the rest of the log line.  A punctuation
         character could be used too.
        </para>
       </tip>

       <tip>
        <para>
         <application>Syslog</> produces its own 
         time stamp and process ID information, so you probably do not want to
         use those escapes if you are logging to <application>syslog</>.
        </para>
       </tip>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-lock-waits" xreflabel="log_lock_waits">
      <term><varname>log_lock_waits</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_lock_waits</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls whether a log message is produced when a session waits
        longer than <xref linkend="guc-deadlock-timeout"> to acquire a
        lock.  This is useful in determining if lock waits are causing
        poor performance.  The default is <literal>off</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-statement" xreflabel="log_statement">
      <term><varname>log_statement</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_statement</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls which SQL statements are logged. Valid values are
        <literal>none</>, <literal>ddl</>, <literal>mod</>, and
        <literal>all</>. <literal>ddl</> logs all data definition
        statements, such as <command>CREATE</>, <command>ALTER</>, and
        <command>DROP</> statements. <literal>mod</> logs all
        <literal>ddl</> statements, plus data-modifying statements
        such as <command>INSERT</>,
        <command>UPDATE</>, <command>DELETE</>, <command>TRUNCATE</>,
        and <command>COPY FROM</>.
        <command>PREPARE</>, <command>EXECUTE</>, and
        <command>EXPLAIN ANALYZE</> statements are also logged if their
        contained command is of an appropriate type.  For clients using
        extended query protocol, logging occurs when an Execute message
        is received, and values of the Bind parameters are included
        (with any embedded single-quote marks doubled).
       </para>

       <para>
        The default is <literal>none</>. Only superusers can change this
        setting.
       </para>

       <note>
        <para>
         Statements that contain simple syntax errors are not logged
         even by the <varname>log_statement</> = <literal>all</> setting,
         because the log message is emitted only after basic parsing has
         been done to determine the statement type.  In the case of extended
         query protocol, this setting likewise does not log statements that
         fail before the Execute phase (i.e., during parse analysis or
         planning).  Set <varname>log_min_error_statement</> to
         <literal>ERROR</> (or lower) to log such statements.      
        </para>
       </note>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-temp-files" xreflabel="log_temp_files">
      <term><varname>log_temp_files</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>log_temp_files</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls whether temporary files are logged when deleted.
        Temporary files can be
        created for sorts, hashes, and temporary query results.
        A value of zero logs all temporary files, and positive
        values log only files whose size is equal or greater than
        the specified number of kilobytes.  The
        default is <literal>-1</>, which disables this logging.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-timezone" xreflabel="log_timezone">
      <term><varname>log_timezone</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>log_timezone</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the time zone used for timestamps written in the log.
        Unlike <xref linkend="guc-timezone">, this value is cluster-wide,
        so that all sessions will report timestamps consistently.
        The default is <literal>unknown</>, which means to use whatever
        the system environment specifies as the time zone.  See <xref
        linkend="datatype-timezones"> for more information.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
     <sect2 id="runtime-config-logging-csvlog">
     <title>Using CSV-Format Log Output</title>

       <para>
        Including <literal>csvlog</> in the <varname>log_destination</> list 
        provides a convenient way to import log files into a database table. 
        This option emits log lines in comma-separated-value format,
        with these columns: timestamp with milliseconds, user name, database
        name, process ID, host:port number, session ID, per-session or -process line
        number, command tag, session start time, virtual transaction ID,
        regular transaction id, error severity, SQL state code, error message,
        error message detail, hint, internal query that led to the error (if
        any), character count of the error position thereof, error context,
        user query that led to the error (if any and enabled by
        <varname>log_min_error_statement</>), character count of the error
        position thereof, location of the error in the PostgreSQL source code
        (if <varname>log_error_verbosity</> is set to <literal>verbose</>).
        Here is a sample table definition for storing CSV-format log output:

<programlisting>
CREATE TABLE postgres_log
(
  log_time timestamp(3) with time zone,
  user_name text,
  database_name text,
  process_id integer,
  connection_from text,
  session_id text,
  session_line_num bigint,
  command_tag text,
  session_start_time timestamp with time zone,
  virtual_transaction_id text,
  transaction_id bigint,
  error_severity text,
  sql_state_code text,
  message text,
  detail text,
  hint text,
  internal_query text,
  internal_query_pos integer,
  context text,
  query text,
  query_pos integer,
  location text,
  PRIMARY KEY (session_id, session_line_num)
);
</programlisting>
       </para>

       <para>
        To import a log file into this table, use the <command>COPY FROM</>
        command:

<programlisting>
COPY postgres_log FROM '/full/path/to/logfile.csv' WITH csv;
</programlisting>
       </para>

       <para>
       There are a few things you need to do to simplify importing CSV log
       files easily and automatically:

       <orderedlist>
         <listitem>
           <para>
            Set <varname>log_filename</varname> and
            <varname>log_rotation_age</> to provide a consistent,
            predictable naming scheme for your log files.  This lets you
            predict what the file name will be and know when an individual log
            file is complete and therefore ready to be imported.
         </para>
        </listitem>

        <listitem>
           <para>
            Set <varname>log_rotation_size</varname> to 0 to disable 
            size-based log rotation, as it makes the log file name difficult 
            to predict. 
           </para>
        </listitem>

        <listitem>
          <para>
           Set <varname>log_truncate_on_rotation</varname> to <literal>on</> so
           that old log data isn't mixed with the new in the same file.
          </para>
        </listitem>

        <listitem>
          <para>
           The table definition above includes a primary key specification.
           This is useful to protect against accidentally importing the same
           information twice.  The <command>COPY</> command commits all of the
           data it imports at one time, so any error will cause the entire
           import to fail.  If you import a partial log file and later import
           the file again when it is complete, the primary key violation will
           cause the import to fail.  Wait until the log is complete and
           closed before importing.  This procedure will also protect against
           accidentally importing a partial line that hasn't been completely
           written, which would also cause <command>COPY</> to fail.
          </para>
        </listitem>
        </orderedlist>
      </para>

    </sect2>
   </sect1>

   <sect1 id="runtime-config-statistics">
    <title>Run-Time Statistics</title>

    <sect2 id="runtime-config-statistics-collector">
     <title>Query and Index Statistics Collector</title>

     <para>
      These parameters control server-wide statistics collection features.
      When statistics collection is enabled, the data that is produced can be
      accessed via the <structname>pg_stat</structname> and
      <structname>pg_statio</structname> family of system views.
      Refer to <xref linkend="monitoring"> for more information.
     </para>

     <variablelist>

     <varlistentry id="guc-track-activities" xreflabel="track_activities">
      <term><varname>track_activities</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>track_activities</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables the collection of information on the currently
        executing command of each session, along with the time at
        which that command began execution. This parameter is on by
        default. Note that even when enabled, this information is not
        visible to all users, only to superusers and the user owning
        the session being reported on; so it should not represent a
        security risk.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-track-counts" xreflabel="track_counts">
      <term><varname>track_counts</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>track_counts</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables collection of statistics on database activity.
        This parameter is on by default, because the autovacuum
        daemon needs the collected information.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-update-process-title" xreflabel="update_process_title">
      <term><varname>update_process_title</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>update_process_title</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Enables updating of the process title every time a new SQL command
        is received by the server.  The process title is typically viewed
        by the <command>ps</> command,
        or in Windows by using the <application>Process Explorer</>.
        Only superusers can change this setting.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

    <sect2 id="runtime-config-statistics-monitor">
     <title>Statistics Monitoring</title>
     <variablelist>

     <varlistentry>
      <term><varname>log_statement_stats</varname> (<type>boolean</type>)</term>
      <term><varname>log_parser_stats</varname> (<type>boolean</type>)</term>
      <term><varname>log_planner_stats</varname> (<type>boolean</type>)</term>
      <term><varname>log_executor_stats</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>log_statement_stats</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>log_parser_stats</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>log_planner_stats</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>log_executor_stats</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        For each query, write performance statistics of the respective
        module to the server log. This is a crude profiling
        instrument.  <varname>log_statement_stats</varname> reports total
        statement statistics, while the others report per-module statistics.
        <varname>log_statement_stats</varname> cannot be enabled together with
        any of the per-module options.  All of these options are disabled by
        default.   Only superusers can change these settings.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect2>
   </sect1>

   <sect1 id="runtime-config-autovacuum">
    <title>Automatic Vacuuming</title>

    <indexterm>
     <primary>autovacuum</primary>
     <secondary>configuration parameters</secondary>
    </indexterm>

     <para>
      These settings control the behavior of the <firstterm>autovacuum</>
      feature.  Refer to <xref linkend="autovacuum"> for
      more information.
     </para>

    <variablelist>

     <varlistentry id="guc-autovacuum" xreflabel="autovacuum">
      <term><varname>autovacuum</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>autovacuum</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls whether the server should run the
        autovacuum launcher daemon.  This is on by default; however,
        <xref linkend="guc-track-counts"> must also be turned on for
        autovacuum to work.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
       <para>
        Note that even when this parameter is disabled, the system
        will launch autovacuum processes if necessary to
        prevent transaction ID wraparound.  See <xref
        linkend="vacuum-for-wraparound"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-log-autovacuum-min-duration" xreflabel="log_autovacuum_min_duration">
      <term><varname>log_autovacuum_min_duration</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>log_autovacuum_min_duration</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Causes each action executed by autovacuum to be logged if it ran for at
        least the specified number of milliseconds.  Setting this to zero logs
        all autovacuum actions. Minus-one (the default) disables logging
        autovacuum actions.  For example, if you set this to
        <literal>250ms</literal> then all automatic vacuums and analyzes that run
        250ms or longer will be logged.  Enabling this parameter can be helpful
        in tracking autovacuum activity.  This setting can only be set in
        the <filename>postgresql.conf</> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-max-workers" xreflabel="autovacuum_max_workers">
      <term><varname>autovacuum_max_workers</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>autovacuum_max_workers</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the maximum number of autovacuum processes (other than the
        autovacuum launcher) which may be running at any one time.  The default
        is three.  This parameter can only be set in
        the <filename>postgresql.conf</> file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-naptime" xreflabel="autovacuum_naptime">
      <term><varname>autovacuum_naptime</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>autovacuum_naptime</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the minimum delay between autovacuum runs on any given
        database.  In each round the daemon examines the
        database and issues <command>VACUUM</> and <command>ANALYZE</> commands
        as needed for tables in that database.  The delay is measured
        in seconds, and the default is one minute (<literal>1m</>).
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-vacuum-threshold" xreflabel="autovacuum_vacuum_threshold">
      <term><varname>autovacuum_vacuum_threshold</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>autovacuum_vacuum_threshold</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the minimum number of updated or deleted tuples needed
        to trigger a <command>VACUUM</> in any one table.
        The default is 50 tuples.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
        This setting can be overridden for individual tables by entries in
        <structname>pg_autovacuum</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-analyze-threshold" xreflabel="autovacuum_analyze_threshold">
      <term><varname>autovacuum_analyze_threshold</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>autovacuum_analyze_threshold</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the minimum number of inserted, updated or deleted tuples
        needed to trigger an <command>ANALYZE</> in any one table.
        The default is 50 tuples.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
        This setting can be overridden for individual tables by entries in
        <structname>pg_autovacuum</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-vacuum-scale-factor" xreflabel="autovacuum_vacuum_scale_factor">
      <term><varname>autovacuum_vacuum_scale_factor</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>autovacuum_vacuum_scale_factor</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies a fraction of the table size to add to
        <varname>autovacuum_vacuum_threshold</varname>
        when deciding whether to trigger a <command>VACUUM</>.
        The default is 0.2 (20% of table size).
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
        This setting can be overridden for individual tables by entries in
        <structname>pg_autovacuum</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-analyze-scale-factor" xreflabel="autovacuum_analyze_scale_factor">
      <term><varname>autovacuum_analyze_scale_factor</varname> (<type>floating point</type>)</term>
      <indexterm>
       <primary><varname>autovacuum_analyze_scale_factor</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies a fraction of the table size to add to
        <varname>autovacuum_analyze_threshold</varname>
        when deciding whether to trigger an <command>ANALYZE</>.
        The default is 0.1 (10% of table size).
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
        This setting can be overridden for individual tables by entries in
        <structname>pg_autovacuum</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-freeze-max-age" xreflabel="autovacuum_freeze_max_age">
      <term><varname>autovacuum_freeze_max_age</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>autovacuum_freeze_max_age</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the maximum age (in transactions) that a table's
        <structname>pg_class</>.<structfield>relfrozenxid</> field can
        attain before a <command>VACUUM</> operation is forced to prevent
        transaction ID wraparound within the table.  Note that the system
        will launch autovacuum processes to prevent wraparound even when
        autovacuum is otherwise disabled.
        The default is 200 million transactions.
        This parameter can only be set at server start, but the setting
        can be reduced for individual tables by entries in
        <structname>pg_autovacuum</>.
        For more information see <xref linkend="vacuum-for-wraparound">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-vacuum-cost-delay" xreflabel="autovacuum_vacuum_cost_delay">
      <term><varname>autovacuum_vacuum_cost_delay</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>autovacuum_vacuum_cost_delay</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the cost delay value that will be used in automatic
        <command>VACUUM</> operations.  If <literal>-1</> is
        specified, the regular
        <xref linkend="guc-vacuum-cost-delay"> value will be used.
        The default value is 20 milliseconds.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
        This setting can be overridden for individual tables by entries in
        <structname>pg_autovacuum</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-autovacuum-vacuum-cost-limit" xreflabel="autovacuum_vacuum_cost_limit">
      <term><varname>autovacuum_vacuum_cost_limit</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>autovacuum_vacuum_cost_limit</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the cost limit value that will be used in automatic
        <command>VACUUM</> operations.  If <literal>-1</> is specified (which is the
        default), the regular
        <xref linkend="guc-vacuum-cost-limit"> value will be used.  Note that
        the value is distributed proportionally among the running autovacuum
        workers, if there is more than one, so that the sum of the limits of
        each worker never exceeds the limit on this variable.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
        This setting can be overridden for individual tables by entries in
        <structname>pg_autovacuum</>.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </sect1>

   <sect1 id="runtime-config-client">
    <title>Client Connection Defaults</title>

    <sect2 id="runtime-config-client-statement">
     <title>Statement Behavior</title>
     <variablelist>

     <varlistentry id="guc-search-path" xreflabel="search_path">
      <term><varname>search_path</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>search_path</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>path</><secondary>for schemas</></>
      <listitem>
       <para>
        This variable specifies the order in which schemas are searched
        when an object (table, data type, function, etc.) is referenced by a
        simple name with no schema component.  When there are objects of
        identical names in different schemas, the one found first
        in the search path is used.  An object that is not in any of the
        schemas in the search path can only be referenced by specifying
        its containing schema with a qualified (dotted) name.
       </para>

       <para>
        The value for <varname>search_path</varname> has to be a comma-separated
        list of schema names.  If one of the list items is
        the special value <literal>$user</literal>, then the schema
        having the name returned by <function>SESSION_USER</> is substituted, if there
        is such a schema.  (If not, <literal>$user</literal> is ignored.)
       </para>

       <para>
        The system catalog schema, <literal>pg_catalog</>, is always
        searched, whether it is mentioned in the path or not.  If it is
        mentioned in the path then it will be searched in the specified
        order.  If <literal>pg_catalog</> is not in the path then it will
        be searched <emphasis>before</> searching any of the path items.
       </para>

       <para>
        Likewise, the current session's temporary-table schema,
        <literal>pg_temp_<replaceable>nnn</></>, is always searched if it
        exists.  It can be explicitly listed in the path by using the
        alias <literal>pg_temp</>.  If it is not listed in the path then
        it is searched first (before even <literal>pg_catalog</>).  However,
        the temporary schema is only searched for relation (table, view,
        sequence, etc) and data type names.  It will never be searched for
        function or operator names.
       </para>

       <para>
        When objects are created without specifying a particular target
        schema, they will be placed in the first schema listed
        in the search path.  An error is reported if the search path is
        empty.
       </para>

       <para>
        The default value for this parameter is
        <literal>'"$user", public'</literal> (where the second part will be
        ignored if there is no schema named <literal>public</>).
        This supports shared use of a database (where no users
        have private schemas, and all share use of <literal>public</>),
        private per-user schemas, and combinations of these.  Other
        effects can be obtained by altering the default search path
        setting, either globally or per-user.
       </para>

       <para>
        The current effective value of the search path can be examined
        via the <acronym>SQL</acronym> function
        <function>current_schemas()</>.  This is not quite the same as
        examining the value of <varname>search_path</varname>, since
        <function>current_schemas()</> shows how the requests
        appearing in <varname>search_path</varname> were resolved.
       </para>

       <para>
        For more information on schema handling, see <xref linkend="ddl-schemas">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-tablespace" xreflabel="default_tablespace">
      <term><varname>default_tablespace</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>default_tablespace</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>tablespace</><secondary>default</></>
      <listitem>
       <para>
        This variable specifies the default tablespace in which to create
        objects (tables and indexes) when a <command>CREATE</> command does
        not explicitly specify a tablespace.
       </para>

       <para>
        The value is either the name of a tablespace, or an empty string
        to specify using the default tablespace of the current database.
        If the value does not match the name of any existing tablespace,
        <productname>PostgreSQL</> will automatically use the default
        tablespace of the current database.  If a nondefault tablespace
        is specified, the user must have <literal>CREATE</> privilege
        for it, or creation attempts will fail.
       </para>

       <para>
        This variable is not used for temporary tables; for them,
        <xref linkend="guc-temp-tablespaces"> is consulted instead.
       </para>

       <para>
        For more information on tablespaces,
        see <xref linkend="manage-ag-tablespaces">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-temp-tablespaces" xreflabel="temp_tablespaces">
      <term><varname>temp_tablespaces</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>temp_tablespaces</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>tablespace</><secondary>temporary</></>
      <listitem>
       <para>
        This variable specifies tablespace(s) in which to create temporary
        objects (temp tables and indexes on temp tables) when a
        <command>CREATE</> command does not explicitly specify a tablespace.
        Temporary files for purposes such as sorting large data sets
        are also created in these tablespace(s).
       </para>

       <para>
        The value is a list of names of tablespaces.  When there is more than
        one name in the list, <productname>PostgreSQL</> chooses a random
        member of the list each time a temporary object is to be created;
        except that within a transaction, successively created temporary
        objects are placed in successive tablespaces from the list.
        If the selected element of the list is an empty string,
        <productname>PostgreSQL</> will automatically use the default
        tablespace of the current database instead.
       </para>

       <para>
        When <varname>temp_tablespaces</> is set interactively, specifying a
        nonexistent tablespace is an error, as is specifying a tablespace for
        which the user does not have <literal>CREATE</> privilege.  However,
        when using a previously set value, nonexistent tablespaces are
        ignored, as are tablespaces for which the user lacks
        <literal>CREATE</> privilege.  In particular, this rule applies when
        using a value set in <filename>postgresql.conf</>.
       </para>

       <para>
        The default value is an empty string, which results in all temporary
        objects being created in the default tablespace of the current
        database.
       </para>

       <para>
        See also <xref linkend="guc-default-tablespace">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-check-function-bodies" xreflabel="check_function_bodies">
      <term><varname>check_function_bodies</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>check_function_bodies</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This parameter is normally on. When set to <literal>off</>, it
        disables validation of the function body string during <xref
        linkend="sql-createfunction"
        endterm="sql-createfunction-title">. Disabling validation is
        occasionally useful to avoid problems such as forward references
        when restoring function definitions from a dump.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-transaction-isolation" xreflabel="default_transaction_isolation">
      <indexterm>
       <primary>transaction isolation level</primary>
      </indexterm>
      <indexterm>
       <primary><varname>default_transaction_isolation</> configuration parameter</primary>
      </indexterm>
      <term><varname>default_transaction_isolation</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        Each SQL transaction has an isolation level, which can be
        either <quote>read uncommitted</quote>, <quote>read
        committed</quote>, <quote>repeatable read</quote>, or
        <quote>serializable</quote>.  This parameter controls the
        default isolation level of each new transaction. The default
        is <quote>read committed</quote>.
       </para>

       <para>
        Consult <xref linkend="mvcc"> and <xref
        linkend="sql-set-transaction"
        endterm="sql-set-transaction-title"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-transaction-read-only" xreflabel="default_transaction_read_only">
      <indexterm>
       <primary>read-only transaction</primary>
      </indexterm>
      <indexterm>
       <primary><varname>default_transaction_read_only</> configuration parameter</primary>
      </indexterm>

      <term><varname>default_transaction_read_only</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        A read-only SQL transaction cannot alter non-temporary tables.
        This parameter controls the default read-only status of each new
        transaction. The default is <literal>off</> (read/write).
       </para>

       <para>
        Consult <xref linkend="sql-set-transaction"
        endterm="sql-set-transaction-title"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-session-replication-role" xreflabel="session_replication_role">
      <term><varname>session_replication_role</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>session_replication_role</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Controls firing of replication-related triggers and rules for the
        current session.  Setting this variable requires
        superuser privilege and results in discarding any previously cached
        query plans.  Possible values are <literal>origin</> (the default),
        <literal>replica</> and <literal>local</>.
        See <xref linkend="sql-altertable" endterm="sql-altertable-title"> for
        more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-statement-timeout" xreflabel="statement_timeout">
      <term><varname>statement_timeout</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>statement_timeout</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Abort any statement that takes over the specified number of
        milliseconds, starting from the time the command arrives at the server
        from the client.  If <varname>log_min_error_statement</> is set to
        <literal>ERROR</> or lower, the statement that timed out will also be
        logged.  A value of zero (the default) turns off the 
        limitation.
       </para>

       <para>
        Setting <varname>statement_timeout</> in
        <filename>postgresql.conf</> is not recommended because it
        affects all sessions.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-vacuum-freeze-min-age" xreflabel="vacuum_freeze_min_age">
      <term><varname>vacuum_freeze_min_age</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>vacuum_freeze_min_age</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Specifies the cutoff age (in transactions) that <command>VACUUM</>
        should use to decide whether to replace transaction IDs with
        <literal>FrozenXID</> while scanning a table.
        The default is 100 million transactions.  Although
        users can set this value anywhere from zero to one billion,
        <command>VACUUM</> will silently limit the effective value to half
        the value of <xref linkend="guc-autovacuum-freeze-max-age">, so
        that there is not an unreasonably short time between forced
        autovacuums.  For more information see <xref
        linkend="vacuum-for-wraparound">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-xmlbinary" xreflabel="xmlbinary">
      <term><varname>xmlbinary</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>xmlbinary</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets how binary values are to be encoded in XML.  This applies
        for example when <type>bytea</type> values are converted to
        XML by the functions <function>xmlelement</function> or
        <function>xmlforest</function>.  Possible values are
        <literal>base64</literal> and <literal>hex</literal>, which
        are both defined in the XML Schema standard.  The default is
        <literal>base64</literal>.  For further information about
        XML-related functions, see <xref linkend="functions-xml">.
       </para>

       <para>
        The actual choice here is mostly a matter of taste,
        constrained only by possible restrictions in client
        applications.  Both methods support all possible values,
        although the hex encoding will be somewhat larger than the
        base64 encoding.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-xmloption" xreflabel="xmloption">
      <term><varname>xmloption</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>xmloption</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><varname>SET XML OPTION</></primary>
      </indexterm>
      <indexterm>
       <primary>XML option</primary>
      </indexterm>
      <listitem>
       <para>
        Sets whether <literal>DOCUMENT</literal> or
        <literal>CONTENT</literal> is implicit when converting between
        XML and character string values.  See <xref
        linkend="datatype-xml"> for a description of this.  Valid
        values are <literal>DOCUMENT</literal> and
        <literal>CONTENT</literal>.  The default is
        <literal>CONTENT</literal>.
       </para>

       <para>
        According to the SQL standard, the command to set this option is
<synopsis>
SET XML OPTION { DOCUMENT | CONTENT };
</synopsis>
        This syntax is also available in PostgreSQL.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
     <sect2 id="runtime-config-client-format">
     <title>Locale and Formatting</title>

     <variablelist>

     <varlistentry id="guc-datestyle" xreflabel="DateStyle">
      <term><varname>DateStyle</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>DateStyle</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the display format for date and time values, as well as the
        rules for interpreting ambiguous date input values. For
        historical reasons, this variable contains two independent
        components: the output format specification (<literal>ISO</>,
        <literal>Postgres</>, <literal>SQL</>, or <literal>German</>)
        and the input/output specification for year/month/day ordering
        (<literal>DMY</>, <literal>MDY</>, or <literal>YMD</>). These
        can be set separately or together. The keywords <literal>Euro</>
        and <literal>European</> are synonyms for <literal>DMY</>; the
        keywords <literal>US</>, <literal>NonEuro</>, and
        <literal>NonEuropean</> are synonyms for <literal>MDY</>. See
        <xref linkend="datatype-datetime"> for more information. The
        built-in default is <literal>ISO, MDY</>, but
        <application>initdb</application> will initialize the
        configuration file with a setting that corresponds to the
        behavior of the chosen <varname>lc_time</varname> locale.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-timezone" xreflabel="timezone">
      <term><varname>timezone</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>timezone</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>time zone</></>
      <listitem>
       <para>
        Sets the time zone for displaying and interpreting time stamps.
        The default is <literal>unknown</>, which means to use whatever
        the system environment specifies as the time zone.  See <xref
        linkend="datatype-timezones"> for more
        information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-timezone-abbreviations" xreflabel="timezone_abbreviations">
      <term><varname>timezone_abbreviations</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>timezone_abbreviations</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>time zone names</></>
      <listitem>
       <para>
        Sets the collection of time zone abbreviations that will be accepted
        by the server for datetime input.  The default is <literal>'Default'</>,
        which is a collection that works in most of the world; there are
        also 'Australia' and 'India', and other collections can be defined
        for a particular installation.  See <xref
        linkend="datetime-appendix"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-extra-float-digits" xreflabel="extra_float_digits">
      <indexterm>
       <primary>significant digits</primary>
      </indexterm>
      <indexterm>
       <primary>floating-point</primary>
       <secondary>display</secondary>
      </indexterm>
      <indexterm>
       <primary><varname>extra_float_digits</> configuration parameter</primary>
      </indexterm>

      <term><varname>extra_float_digits</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        This parameter adjusts the number of digits displayed for
        floating-point values, including <type>float4</>, <type>float8</>,
        and geometric data types.  The parameter value is added to the
        standard number of digits (<literal>FLT_DIG</> or <literal>DBL_DIG</>
        as appropriate).  The value can be set as high as 2, to include
        partially-significant digits; this is especially useful for dumping
        float data that needs to be restored exactly.  Or it can be set
        negative to suppress unwanted digits.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-client-encoding" xreflabel="client_encoding">
      <term><varname>client_encoding</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>client_encoding</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>character set</></>
      <listitem>
       <para>
        Sets the client-side encoding (character set).
        The default is to use the database encoding.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-messages" xreflabel="lc_messages">
      <term><varname>lc_messages</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_messages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the language in which messages are displayed.  Acceptable
        values are system-dependent; see <xref linkend="locale"> for
        more information.  If this variable is set to the empty string
        (which is the default) then the value is inherited from the
        execution environment of the server in a system-dependent way.
       </para>

       <para>
        On some systems, this locale category does not exist.  Setting
        this variable will still work, but there will be no effect.
        Also, there is a chance that no translated messages for the
        desired language exist.  In that case you will continue to see
        the English messages.
       </para>

       <para>
        Only superusers can change this setting, because it affects the
        messages sent to the server log as well as to the client.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-monetary" xreflabel="lc_monetary">
      <term><varname>lc_monetary</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_monetary</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the locale to use for formatting monetary amounts, for
        example with the <function>to_char</function> family of
        functions.  Acceptable values are system-dependent; see <xref
        linkend="locale"> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-numeric" xreflabel="lc_numeric">
      <term><varname>lc_numeric</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_numeric</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the locale to use for formatting numbers, for example
        with the <function>to_char</function> family of
        functions. Acceptable values are system-dependent; see <xref
        linkend="locale"> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-time" xreflabel="lc_time">
      <term><varname>lc_time</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_time</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Sets the locale to use for formatting date and time values.
        (Currently, this setting does nothing, but it might in the
        future.)  Acceptable values are system-dependent; see <xref
        linkend="locale"> for more information.  If this variable is
        set to the empty string (which is the default) then the value
        is inherited from the execution environment of the server in a
        system-dependent way.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-text-search-config" xreflabel="default_text_search_config">
      <term><varname>default_text_search_config</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>default_text_search_config</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Selects the text search configuration that is used by those variants
        of the text search functions that do not have an explicit argument
        specifying the configuration.
        See <xref linkend="textsearch"> for further information.
        The built-in default is <literal>pg_catalog.simple</>, but
        <application>initdb</application> will initialize the
        configuration file with a setting that corresponds to the
        chosen <varname>lc_ctype</varname> locale, if a configuration
        matching that locale can be identified.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>

    </sect2>
     <sect2 id="runtime-config-client-other">
     <title>Other Defaults</title>

     <variablelist>

     <varlistentry id="guc-dynamic-library-path" xreflabel="dynamic_library_path">
      <term><varname>dynamic_library_path</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>dynamic_library_path</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>dynamic loading</></>
      <listitem>
       <para>
        If a dynamically loadable module needs to be opened and the
        file name specified in the <command>CREATE FUNCTION</command> or
        <command>LOAD</command> command
        does not have a directory component (i.e. the
        name does not contain a slash), the system will search this
        path for the required file.
       </para>

       <para>
        The value for <varname>dynamic_library_path</varname> has to be a
        list of absolute directory paths separated by colons (or semi-colons
        on Windows).  If a list element starts
        with the special string <literal>$libdir</literal>, the
        compiled-in <productname>PostgreSQL</productname> package
        library directory is substituted for <literal>$libdir</literal>. This
        is where the modules provided by the standard
        <productname>PostgreSQL</productname> distribution are installed.
        (Use <literal>pg_config --pkglibdir</literal> to find out the name of
        this directory.) For example:
<programlisting>
dynamic_library_path = '/usr/local/lib/postgresql:/home/my_project/lib:$libdir'
</programlisting>
        or, in a Windows environment:
<programlisting>
dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
</programlisting>
       </para>

       <para>
        The default value for this parameter is
        <literal>'$libdir'</literal>. If the value is set to an empty
        string, the automatic path search is turned off.
       </para>

       <para>
        This parameter can be changed at run time by superusers, but a
        setting done that way will only persist until the end of the
        client connection, so this method should be reserved for
        development purposes. The recommended way to set this parameter
        is in the <filename>postgresql.conf</filename> configuration
        file.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-gin-fuzzy-search-limit" xreflabel="gin_fuzzy_search_limit">
      <term><varname>gin_fuzzy_search_limit</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>gin_fuzzy_search_limit</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Soft upper limit of the size of the set returned by GIN index. For more
        information see <xref linkend="gin-tips">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-local-preload-libraries" xreflabel="local_preload_libraries">
      <term><varname>local_preload_libraries</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>local_preload_libraries</> configuration parameter</primary>
      </indexterm>
      <indexterm>
       <primary><filename>$libdir/plugins</></primary>
      </indexterm>
      <listitem>
       <para>
        This variable specifies one or more shared libraries that are
        to be preloaded at connection start.  If more than one library
        is to be loaded, separate their names with commas.
        This parameter cannot be changed after the start of a particular
        session.
       </para>

       <para>
        Because this is not a superuser-only option, the libraries
        that can be loaded are restricted to those appearing in the
        <filename>plugins</> subdirectory of the installation's
        standard library directory.  (It is the database administrator's
        responsibility to ensure that only <quote>safe</> libraries
        are installed there.)  Entries in <varname>local_preload_libraries</>
        can specify this directory explicitly, for example
        <literal>$libdir/plugins/mylib</literal>, or just specify
        the library name &mdash; <literal>mylib</literal> would have
        the same effect as <literal>$libdir/plugins/mylib</literal>.
       </para>

       <para>
        There is no performance advantage to loading a library at session
        start rather than when it is first used.  Rather, the intent of
        this feature is to allow debugging or performance-measurement
        libraries to be loaded into specific sessions without an explicit
        <command>LOAD</> command being given.  For example, debugging could
        be enabled for all sessions under a given user name by setting
        this parameter with <command>ALTER USER SET</>.
       </para>

       <para>
        If a specified library is not found,
        the connection attempt will fail.
       </para>

       <para>
        Every  PostgreSQL-supported library has a <quote>magic
        block</> that is checked to guarantee compatibility.  
        For this reason, non-PostgreSQL libraries cannot be
        loaded in this way.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-locks">
    <title>Lock Management</title>

     <variablelist>

     <varlistentry id="guc-deadlock-timeout" xreflabel="deadlock_timeout">
      <indexterm>
       <primary>deadlock</primary>
       <secondary>timeout during</secondary>
      </indexterm>
      <indexterm>
       <primary>timeout</primary>
       <secondary>deadlock</secondary>
      </indexterm>
      <indexterm>
       <primary><varname>deadlock_timeout</> configuration parameter</primary>
      </indexterm>

      <term><varname>deadlock_timeout</varname> (<type>integer</type>)</term>
      <listitem>
       <para>
        This is the amount of time, in milliseconds, to wait on a lock
        before checking to see if there is a deadlock condition. The
        check for deadlock is relatively slow, so the server doesn't run
        it every time it waits for a lock. We optimistically assume
        that deadlocks are not common in production applications and
        just wait on the lock for a while before starting the check for a
        deadlock. Increasing this value reduces the amount of time
        wasted in needless deadlock checks, but slows down reporting of
        real deadlock errors. The default is one second (<literal>1s</>),
        which is probably about the smallest value you would want in
        practice. On a heavily loaded server you might want to raise it.
        Ideally the setting should exceed your typical transaction time,
        so as to improve the odds that a lock will be released before
        the waiter decides to check for deadlock.
       </para>

       <para>
        When <xref linkend="guc-log-lock-waits"> is set,
        this parameter also determines the length of time to wait before
        a log message is issued about the lock wait.  If you are trying
        to investigate locking delays you might want to set a shorter than
        normal <varname>deadlock_timeout</varname>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-locks-per-transaction" xreflabel="max_locks_per_transaction">
      <term><varname>max_locks_per_transaction</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_locks_per_transaction</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The shared lock table is created to track locks on
        <varname>max_locks_per_transaction</varname> * (<xref
        linkend="guc-max-connections"> + <xref
        linkend="guc-max-prepared-transactions">) objects (e.g.  tables);
        hence, no more than this many distinct objects can be locked at
        any one time.  This parameter controls the average number of object
        locks allocated for each transaction;  individual transactions
        can lock more objects as long as the locks of all transactions
        fit in the lock table.  This is <emphasis>not</> the number of
        rows that can be locked; that value is unlimited.  The default,
        64, has historically proven sufficient, but you might need to
        raise this value if you have clients that touch many different
        tables in a single transaction. This parameter can only be set at
        server start.
       </para>

       <para>
        Increasing this parameter might cause <productname>PostgreSQL</>
        to request more <systemitem class="osname">System V</> shared
        memory than your operating system's default configuration
        allows. See <xref linkend="sysvipc"> for information on how to
        adjust those parameters, if necessary.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
   </sect1>

   <sect1 id="runtime-config-compatible">
    <title>Version and Platform Compatibility</title>

    <sect2 id="runtime-config-compatible-version">
     <title>Previous PostgreSQL Versions</title>

     <variablelist>

     <varlistentry id="guc-add-missing-from" xreflabel="add_missing_from">
      <term><varname>add_missing_from</varname> (<type>boolean</type>)</term>
      <indexterm><primary>FROM</><secondary>missing</></>
      <indexterm>
       <primary><varname>add_missing_from</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When on, tables that are referenced by a query will be
        automatically added to the <literal>FROM</> clause if not
        already present. This behavior does not comply with the SQL
        standard and many people dislike it because it can mask mistakes
        (such as referencing a table where you should have referenced
        its alias). The default is <literal>off</>. This variable can be
        enabled for compatibility with releases of
        <productname>PostgreSQL</> prior to 8.1, where this behavior was
        allowed by default.
       </para>

       <para>
        Note that even when this variable is enabled, a warning
        message will be emitted for each implicit <literal>FROM</>
        entry referenced by a query. Users are encouraged to update
        their applications to not rely on this behavior, by adding all
        tables referenced by a query to the query's <literal>FROM</>
        clause (or its <literal>USING</> clause in the case of
        <command>DELETE</>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-array-nulls" xreflabel="array_nulls">
      <term><varname>array_nulls</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>array_nulls</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This controls whether the array input parser recognizes
        unquoted <literal>NULL</> as specifying a null array element.
        By default, this is <literal>on</>, allowing array values containing
        null values to be entered.  However, <productname>PostgreSQL</> versions
        before 8.2 did not support null values in arrays, and therefore would
        treat <literal>NULL</> as specifying a normal array element with
        the string value <quote>NULL</>.  For backwards compatibility with
        applications that require the old behavior, this variable can be
        turned <literal>off</>.
       </para>

       <para>
        Note that it is possible to create array values containing null values
        even when this variable is <literal>off</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-backslash-quote" xreflabel="backslash_quote">
      <term><varname>backslash_quote</varname> (<type>string</type>)</term>
      <indexterm><primary>strings</><secondary>backslash quotes</></>
      <indexterm>
       <primary><varname>backslash_quote</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This controls whether a quote mark can be represented by
        <literal>\'</> in a string literal.  The preferred, SQL-standard way
        to represent a quote mark is by doubling it (<literal>''</>) but
        <productname>PostgreSQL</> has historically also accepted
        <literal>\'</>. However, use of <literal>\'</> creates security risks
        because in some client character set encodings, there are multibyte
        characters in which the last byte is numerically equivalent to ASCII
        <literal>\</>.  If client-side code does escaping incorrectly then a
        SQL-injection attack is possible.  This risk can be prevented by
        making the server reject queries in which a quote mark appears to be
        escaped by a backslash.
        The allowed values of <varname>backslash_quote</> are
        <literal>on</> (allow <literal>\'</> always),
        <literal>off</> (reject always), and
        <literal>safe_encoding</> (allow only if client encoding does not
        allow ASCII <literal>\</> within a multibyte character).
        <literal>safe_encoding</> is the default setting.
       </para>

       <para>
        Note that in a standard-conforming string literal, <literal>\</> just
        means <literal>\</> anyway.  This parameter affects the handling of
        non-standard-conforming literals, including
        escape string syntax (<literal>E'...'</>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-default-with-oids" xreflabel="default_with_oids">
      <term><varname>default_with_oids</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>default_with_oids</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This controls whether <command>CREATE TABLE</command> and
        <command>CREATE TABLE AS</command> include an OID column in
        newly-created tables, if neither <literal>WITH OIDS</literal>
        nor <literal>WITHOUT OIDS</literal> is specified. It also
        determines whether OIDs will be included in tables created by
        <command>SELECT INTO</command>. In <productname>PostgreSQL</>
        8.1 <varname>default_with_oids</> is <literal>off</> by default; in
        prior versions of <productname>PostgreSQL</productname>, it
        was on by default.
       </para>

       <para>
        The use of OIDs in user tables is considered deprecated, so
        most installations should leave this variable disabled.
        Applications that require OIDs for a particular table should
        specify <literal>WITH OIDS</literal> when creating the
        table. This variable can be enabled for compatibility with old
        applications that do not follow this behavior.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-escape-string-warning" xreflabel="escape_string_warning">
      <term><varname>escape_string_warning</varname> (<type>boolean</type>)</term>
      <indexterm><primary>strings</><secondary>escape warning</></>
      <indexterm>
       <primary><varname>escape_string_warning</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When on, a warning is issued if a backslash (<literal>\</>)
        appears in an ordinary string literal (<literal>'...'</>
        syntax) and <varname>standard_conforming_strings</varname> is off.
        The default is <literal>on</>.
       </para>
       <para>
        Applications that wish to use backslash as escape should be
        modified to use escape string syntax (<literal>E'...'</>),
        because the default behavior of ordinary strings will change
        in a future release for SQL compatibility.  This variable can
        be enabled to help detect applications that will break.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-regex-flavor" xreflabel="regex_flavor">
      <term><varname>regex_flavor</varname> (<type>string</type>)</term>
      <indexterm><primary>regular expressions</></>
      <indexterm>
       <primary><varname>regex_flavor</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        The regular expression <quote>flavor</> can be set to
        <literal>advanced</>, <literal>extended</>, or <literal>basic</>.
        The default is <literal>advanced</>.  The <literal>extended</>
        setting might be useful for exact backwards compatibility with
        pre-7.4 releases of <productname>PostgreSQL</>.  See
        <xref linkend="posix-syntax-details"> for details.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-sql-inheritance" xreflabel="sql_inheritance">
      <term><varname>sql_inheritance</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>sql_inheritance</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>inheritance</></>
      <listitem>
       <para>
        This controls the inheritance semantics.  If turned <literal>off</>,
        subtables are not included by various commands by default; basically
        an implied <literal>ONLY</literal> key word.  This was added for 
        compatibility with releases prior to 7.1.  See
        <xref linkend="ddl-inherit"> for more information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-standard-conforming-strings" xreflabel="standard_conforming_strings">
      <term><varname>standard_conforming_strings</varname> (<type>boolean</type>)</term>
      <indexterm><primary>strings</><secondary>standard conforming</></>
      <indexterm>
       <primary><varname>standard_conforming_strings</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This controls whether ordinary string literals
        (<literal>'...'</>) treat backslashes literally, as specified in
        the SQL standard.
        The default is currently <literal>off</>, causing
        <productname>PostgreSQL</productname> to have its historical
        behavior of treating backslashes as escape characters.
        The default will change to <literal>on</> in a future release
        to improve compatibility with the standard.
        Applications can check this
        parameter to determine how string literals will be processed.
        The presence of this parameter can also be taken as an indication
        that the escape string syntax (<literal>E'...'</>) is supported.
        Escape string syntax should be used if an application desires
        backslashes to be treated as escape characters.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-synchronize-seqscans" xreflabel="synchronize_seqscans">
      <term><varname>synchronize_seqscans</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>synchronize_seqscans</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This allows sequential scans of large tables to synchronize with each
        other, so that concurrent scans read the same block at about the
        same time and hence share the I/O workload.  When this is enabled,
        a scan might start in the middle of the table and then <quote>wrap
        around</> the end to cover all rows, so as to synchronize with the
        activity of scans already in progress.  This can result in
        unpredictable changes in the row ordering returned by queries that
        have no <literal>ORDER BY</> clause.  Setting this parameter to
        <literal>off</> ensures the pre-8.3 behavior in which a sequential
        scan always starts from the beginning of the table.  The default
        is <literal>on</>.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>

    <sect2 id="runtime-config-compatible-clients">
     <title>Platform and Client Compatibility</title>
     <variablelist>

     <varlistentry id="guc-transform-null-equals" xreflabel="transform_null_equals">
      <term><varname>transform_null_equals</varname> (<type>boolean</type>)</term>
      <indexterm><primary>IS NULL</></>
      <indexterm>
       <primary><varname>transform_null_equals</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        When on, expressions of the form <literal><replaceable>expr</> =
        NULL</literal> (or <literal>NULL =
        <replaceable>expr</></literal>) are treated as
        <literal><replaceable>expr</> IS NULL</literal>, that is, they
        return true if <replaceable>expr</> evaluates to the null value,
        and false otherwise. The correct SQL-spec-compliant behavior of
        <literal><replaceable>expr</> = NULL</literal> is to always
        return null (unknown). Therefore this parameter defaults to
        <literal>off</>.
       </para>

       <para>
        However, filtered forms in <productname>Microsoft
        Access</productname> generate queries that appear to use
        <literal><replaceable>expr</> = NULL</literal> to test for
        null values, so if you use that interface to access the database you
        might want to turn this option on.  Since expressions of the
        form <literal><replaceable>expr</> = NULL</literal> always
        return the null value (using the correct interpretation) they are not
        very useful and do not appear often in normal applications, so
        this option does little harm in practice.  But new users are
        frequently confused about the semantics of expressions
        involving null values, so this option is not on by default.
       </para>

       <para>
        Note that this option only affects the exact form <literal>= NULL</>,
        not other comparison operators or other expressions
        that are computationally equivalent to some expression
        involving the equals operator (such as <literal>IN</literal>).
        Thus, this option is not a general fix for bad programming.
       </para>

       <para>
        Refer to <xref linkend="functions-comparison"> for related information.
       </para>
      </listitem>
     </varlistentry>

     </variablelist>
    </sect2>
   </sect1>

   <sect1 id="runtime-config-preset">
    <title>Preset Options</title>

    <para>
     The following <quote>parameters</> are read-only, and are determined
     when <productname>PostgreSQL</productname> is compiled or when it is
     installed. As such, they have been excluded from the sample
     <filename>postgresql.conf</> file.  These options report
     various aspects of <productname>PostgreSQL</productname> behavior
     that might be of interest to certain applications, particularly
     administrative front-ends.
    </para>

    <variablelist>

     <varlistentry id="guc-block-size" xreflabel="block_size">
      <term><varname>block_size</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>block_size</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Reports the size of a disk block.  It is determined by the value
        of <literal>BLCKSZ</> when building the server. The default
        value is 8192 bytes.  The meaning of some configuration
        variables (such as <xref linkend="guc-shared-buffers">) is
        influenced by <varname>block_size</varname>. See <xref
        linkend="runtime-config-resource"> for information.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-integer-datetimes" xreflabel="integer_datetimes">
      <term><varname>integer_datetimes</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>integer_datetimes</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Reports whether <productname>PostgreSQL</> was built with
        support for 64-bit-integer dates and times.  This can be
        disabled by configuring with <literal>--disable-integer-datetimes</>
        when building <productname>PostgreSQL</>.  The default value is
        <literal>on</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-collate" xreflabel="lc_collate">
      <term><varname>lc_collate</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_collate</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Reports the locale in which sorting of textual data is done.
        See <xref linkend="locale"> for more information.
        The value is determined when the database cluster is initialized.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-lc-ctype" xreflabel="lc_ctype">
      <term><varname>lc_ctype</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>lc_ctype</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Reports the locale that determines character classifications.
        See <xref linkend="locale"> for more information.
        The value is determined when the database cluster is initialized.
        Ordinarily this will be the same as <varname>lc_collate</varname>,
        but for special applications it might be set differently.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-function-args" xreflabel="max_function_args">
      <term><varname>max_function_args</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_function_args</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Reports the maximum number of function arguments. It is determined by
        the value of <literal>FUNC_MAX_ARGS</> when building the server. The
        default value is 100 arguments.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-identifier-length" xreflabel="max_identifier_length">
      <term><varname>max_identifier_length</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_identifier_length</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Reports the maximum identifier length. It is determined as one
        less than the value of <literal>NAMEDATALEN</> when building
        the server. The default value of <literal>NAMEDATALEN</> is
        64; therefore the default
        <varname>max_identifier_length</varname> is 63 bytes.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-max-index-keys" xreflabel="max_index_keys">
      <term><varname>max_index_keys</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>max_index_keys</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Reports the maximum number of index keys. It is determined by
        the value of <literal>INDEX_MAX_KEYS</> when building the server. The
        default value is 32 keys.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-server-encoding" xreflabel="server_encoding">
      <term><varname>server_encoding</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>server_encoding</> configuration parameter</primary>
      </indexterm>
      <indexterm><primary>character set</></>
      <listitem>
       <para>
        Reports the database encoding (character set).
        It is determined when the database is created.  Ordinarily,
        clients need only be concerned with the value of <xref
        linkend="guc-client-encoding">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-server-version" xreflabel="server_version">
      <term><varname>server_version</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>server_version</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Reports the version number of the server. It is determined by the
        value of <literal>PG_VERSION</> when building the server.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-server-version-num" xreflabel="server_version_num">
      <term><varname>server_version_num</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>server_version_num</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Reports the version number of the server as an integer. It is determined 
        by the value of <literal>PG_VERSION_NUM</> when building the server.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </sect1>

   <sect1 id="runtime-config-custom">
    <title>Customized Options</title>

    <para>
     This feature was designed to allow parameters not normally known to
     <productname>PostgreSQL</productname> to be added by add-on modules
     (such as procedural languages).  This allows add-on modules to be
     configured in the standard ways.
    </para>

    <variablelist>

     <varlistentry id="guc-custom-variable-classes" xreflabel="custom_variable_classes">
      <term><varname>custom_variable_classes</varname> (<type>string</type>)</term>
      <indexterm>
       <primary><varname>custom_variable_classes</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        This variable specifies one or several class names to be used for
        custom variables, in the form of a comma-separated list. A custom
        variable is a variable not normally known
        to <productname>PostgreSQL</productname> proper but used by some
        add-on module.  Such variables must have names consisting of a class
        name, a dot, and a variable name.  <varname>custom_variable_classes</>
        specifies all the class names in use in a particular installation.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>

      </listitem>
     </varlistentry>
    </variablelist>

    <para>
     The difficulty with setting custom variables in
     <filename>postgresql.conf</> is that the file must be read before add-on
     modules have been loaded, and so custom variables would ordinarily be
     rejected as unknown.  When <varname>custom_variable_classes</> is set,
     the server will accept definitions of arbitrary variables within each
     specified class.  These variables will be treated as placeholders and
     will have no function until the module that defines them is loaded. When a
     module for a specific class is loaded, it will add the proper variable
     definitions for its class name, convert any placeholder
     values according to those definitions, and issue warnings for any
     placeholders of its class that remain (which presumably would be
     misspelled configuration variables).
    </para>

    <para>
     Here is an example of what <filename>postgresql.conf</> might contain
     when using custom variables:

<programlisting>
custom_variable_classes = 'plr,plperl'
plr.path = '/usr/lib/R'
plperl.use_strict = true
plruby.use_strict = true        # generates error: unknown class name
</programlisting>
    </para>
   </sect1>

   <sect1 id="runtime-config-developer">
    <title>Developer Options</title>

    <para>
     The following parameters are intended for work on the
     <productname>PostgreSQL</productname> source, and in some cases
     to assist with recovery of severely damaged databases.  There
     should be no reason to use them in a production database setup.
     As such, they have been excluded from the sample
     <filename>postgresql.conf</> file.  Note that many of these
     parameters require special source compilation flags to work at all.
    </para>

    <variablelist>
     <varlistentry id="guc-allow-system-table-mods" xreflabel="allow_system_table_mods">
      <term><varname>allow_system_table_mods</varname> (<type>boolean</type>)</term>
      <indexterm>
        <primary><varname>allow_system_table_mods</varname> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Allows modification of the structure of system tables.
        This is used by <command>initdb</command>.
        This parameter can only be set at server start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-debug-assertions" xreflabel="debug_assertions">
      <term><varname>debug_assertions</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>debug_assertions</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Turns on various assertion checks. This is a debugging aid. If
        you are experiencing strange problems or crashes you might want
        to turn this on, as it might expose programming mistakes. To use
        this parameter, the macro <symbol>USE_ASSERT_CHECKING</symbol>
        must be defined when <productname>PostgreSQL</productname> is
        built (accomplished by the <command>configure</command> option
        <option>--enable-cassert</option>). Note that
        <varname>debug_assertions</varname> defaults to <literal>on</>
        if <productname>PostgreSQL</productname> has been built with
        assertions enabled.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-ignore-system-indexes" xreflabel="ignore_system_indexes">
      <term><varname>ignore_system_indexes</varname> (<type>boolean</type>)</term>
      <indexterm>
        <primary><varname>ignore_system_indexes</varname> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Ignore system indexes when reading system tables (but still
        update the indexes when modifying the tables).  This is useful
        when recovering from damaged system indexes.
        This parameter cannot be changed after session start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-post-auth-delay" xreflabel="post_auth_delay">
      <term><varname>post_auth_delay</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>post_auth_delay</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        If nonzero, a delay of this many seconds occurs when a new
        server process is started, after it conducts the
        authentication procedure.  This is intended to give an
        opportunity to attach to the server process with a debugger.
        This parameter cannot be changed after session start.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-pre-auth-delay" xreflabel="pre_auth_delay">
      <term><varname>pre_auth_delay</varname> (<type>integer</type>)</term>
      <indexterm>
       <primary><varname>pre_auth_delay</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        If nonzero, a delay of this many seconds occurs just after a
        new server process is forked, before it conducts the
        authentication procedure.  This is intended to give an
        opportunity to attach to the server process with a debugger to
        trace down misbehavior in authentication.
        This parameter can only be set in the <filename>postgresql.conf</>
        file or on the server command line.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-trace-notify" xreflabel="trace_notify">
      <term><varname>trace_notify</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>trace_notify</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Generates a great amount of debugging output for the
        <command>LISTEN</command> and <command>NOTIFY</command>
        commands.  <xref linkend="guc-client-min-messages"> or
        <xref linkend="guc-log-min-messages"> must be
        <literal>DEBUG1</literal> or lower to send this output to the
        client or server log, respectively.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-trace-sort" xreflabel="trace_sort">
      <term><varname>trace_sort</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>trace_sort</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        If on, emit information about resource usage during sort operations.
        This parameter is only available if the <symbol>TRACE_SORT</symbol> macro
        was defined when <productname>PostgreSQL</productname> was compiled.
        (However, <symbol>TRACE_SORT</symbol> is currently defined by default.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><varname>trace_locks</varname> (<type>boolean</type>)</term>
      <term><varname>trace_lwlocks</varname> (<type>boolean</type>)</term>
      <term><varname>trace_userlocks</varname> (<type>boolean</type>)</term>
      <term><varname>trace_lock_oidmin</varname> (<type>boolean</type>)</term>
      <term><varname>trace_lock_table</varname> (<type>boolean</type>)</term>
      <term><varname>debug_deadlocks</varname> (<type>boolean</type>)</term>
      <term><varname>log_btree_build_stats</varname> (<type>boolean</type>)</term>
      <listitem>
       <para>
        Various other code tracing and debugging options.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="guc-wal-debug" xreflabel="wal_debug">
      <term><varname>wal_debug</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>wal_debug</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        If on, emit WAL-related debugging output. This parameter is
        only available if the <symbol>WAL_DEBUG</symbol> macro was
        defined when <productname>PostgreSQL</productname> was
        compiled.
       </para>
      </listitem>
     </varlistentry>

    <varlistentry id="guc-zero-damaged-pages" xreflabel="zero_damaged_pages">
      <term><varname>zero_damaged_pages</varname> (<type>boolean</type>)</term>
      <indexterm>
       <primary><varname>zero_damaged_pages</> configuration parameter</primary>
      </indexterm>
      <listitem>
       <para>
        Detection of a damaged page header normally causes
        <productname>PostgreSQL</> to report an error, aborting the current
        command.  Setting <varname>zero_damaged_pages</> to on causes
        the system to instead report a warning, zero out the damaged page,
        and continue processing.  This behavior <emphasis>will destroy data</>,
        namely all the rows on the damaged page.  But it allows you to get
        past the error and retrieve rows from any undamaged pages that might
        be present in the table.  So it is useful for recovering data if
        corruption has occurred due to hardware or software error.  You should
        generally not set this on until you have given up hope of recovering
        data from the damaged page(s) of a table.  The
        default setting is <literal>off</>, and it can only be changed
        by a superuser.
       </para>
      </listitem>
     </varlistentry>
   </variablelist>
  </sect1>
  <sect1 id="runtime-config-short">
   <title>Short Options</title>

   <para>
    For convenience there are also single letter command-line option
    switches available for some parameters.  They are described in
    <xref linkend="runtime-config-short-table">.  Some of these
    options exist for historical reasons, and their presence as a
    single-letter option does not necessarily indicate an endorsement
    to use the option heavily.
   </para>

    <table id="runtime-config-short-table">
     <title>Short option key</title>
     <tgroup cols="2">
      <thead>
       <row>
        <entry>Short option</entry>
        <entry>Equivalent</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><option>-A <replaceable>x</replaceable></option></entry>
        <entry><literal>debug_assertions = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-B <replaceable>x</replaceable></option></entry>
        <entry><literal>shared_buffers = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-d <replaceable>x</replaceable></option></entry>
        <entry><literal>log_min_messages = DEBUG<replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-e</option></entry>
        <entry><literal>datestyle = euro</></entry>
       </row>
       <row>
        <entry>
          <option>-fb</option>, <option>-fh</option>, <option>-fi</option>,
          <option>-fm</option>, <option>-fn</option>,
          <option>-fs</option>, <option>-ft</option>
         </entry>
         <entry>
          <literal>enable_bitmapscan = off</>,
          <literal>enable_hashjoin = off</>,
          <literal>enable_indexscan = off</>,
          <literal>enable_mergejoin = off</>,
          <literal>enable_nestloop = off</>,
          <literal>enable_seqscan = off</>,
          <literal>enable_tidscan = off</>
         </entry>
       </row>
       <row>
        <entry><option>-F</option></entry>
        <entry><literal>fsync = off</></entry>
       </row>
       <row>
        <entry><option>-h <replaceable>x</replaceable></option></entry>
        <entry><literal>listen_addresses = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-i</option></entry>
        <entry><literal>listen_addresses = '*'</></entry>
       </row>
       <row>
        <entry><option>-k <replaceable>x</replaceable></option></entry>
        <entry><literal>unix_socket_directory = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-l</option></entry>
        <entry><literal>ssl = on</></entry>
       </row>
       <row>
        <entry><option>-N <replaceable>x</replaceable></option></entry>
        <entry><literal>max_connections = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-O</option></entry>
        <entry><literal>allow_system_table_mods = on</></entry>
       </row>
       <row>
        <entry><option>-p <replaceable>x</replaceable></option></entry>
        <entry><literal>port = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-P</option></entry>
        <entry><literal>ignore_system_indexes = on</></entry>
       </row>
       <row>
        <entry><option>-s</option></entry>
        <entry><literal>log_statement_stats = on</></entry>
       </row>
       <row>
        <entry><option>-S <replaceable>x</replaceable></option></entry>
        <entry><literal>work_mem = <replaceable>x</replaceable></></entry>
       </row>
       <row>
        <entry><option>-tpa</option>, <option>-tpl</option>, <option>-te</option></entry>
        <entry><literal>log_parser_stats = on</>,
        <literal>log_planner_stats = on</>, 
        <literal>log_executor_stats = on</></entry>
       </row>
       <row>
        <entry><option>-W <replaceable>x</replaceable></option></entry>
        <entry><literal>post_auth_delay = <replaceable>x</replaceable></></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

  </sect1>
</chapter>