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<chapter id="managing-databases">
<para>
A database is a named collection of SQL objects (<quote>database
- objects</quote>); every database object (tables, function, etc.)
- belongs to one and only one database. An application that connects
- to the database server specifies with its connection request the
+ objects</quote>). Generally, every database object (tables, functions,
+ etc.) belongs to one and only one database. (But there are a few system
+ catalogs, for example <literal>pg_database</>, that belong to a whole
+ installation and are accessible from each database within the
+ installation.)
+ An application that connects
+ to the database server specifies in its connection request the
name of the database it wants to connect to. It is not possible to
access more than one database per connection. (But an application
is not restricted in the number of connections it opens to the same
<synopsis>
CREATE DATABASE <replaceable>name</>
</synopsis>
- where <replaceable>name</> can be chosen freely. (Depending on the
+ where <replaceable>name</> follows the usual rules for SQL identifiers.
+ (Depending on the
current implementation, certain characters that are special to the
underlying operating system might be prohibited. There will be
run-time checks for that.) The current user automatically becomes
<para>
The creation of databases is a restricted operation. See <xref
- linkend="user-attributes"> how to grant permission.
+ linkend="user-attributes"> for how to grant permission.
</para>
<formalpara>
question remains how the <emphasis>first</> database at any given
site can be created. The first database is always created by the
<command>initdb</> command when the data storage area is
- initialized. (See <xref linkend="creating-cluster">.) This
- database is called <literal>template1</> and cannot be deleted. So
+ initialized. (See <xref linkend="creating-cluster">.) By convention
+ this database is called <literal>template1</>. So
to create the first <quote>real</> database you can connect to
<literal>template1</>.
</para>
This means that any changes you make in <literal>template1</> are
propagated to all subsequently created databases. This implies that
you should not use the template database for real work, but when
- used judiciously this feature can be convenient.
+ used judiciously this feature can be convenient. More details appear
+ below.
</para>
<para>
</synopsis>
<command>createdb</> does no magic. It connects to the template1
- database and executes the <command>CREATE DATABASE</> command,
- exactly as described above. It uses <application>psql</> program
+ database and issues the <command>CREATE DATABASE</> command,
+ exactly as described above. It uses
the <application>psql</> program
internally. The reference page on <command>createdb</> contains the invocation
- details. In particular, <command>createdb</> without any arguments will create
+ details. Note that <command>createdb</> without any arguments will create
a database with the current user name, which may or may not be what
you want.
</para>
- <sect2>
+ <sect2 id="manage-ag-templatedbs">
+ <title>Template Databases</title>
+
+ <para>
+ <command>CREATE DATABASE</> actually works by copying an existing
+ database. By default, it copies the standard system database named
+ <literal>template1</>. Thus that database is the <quote>template</>
+ from which new databases are made. If you add objects to
+ <literal>template1</>, these objects
+ will be copied into subsequently created user databases. This
+ behavior allows site-local modifications to the standard set of
+ objects in databases. For example, if you install the procedural
+ language <literal>plpgsql</> in <literal>template1</>, it will
+ automatically be available in user databases without any extra action
+ being taken when those databases are made.
+ </para>
+
+ <para>
+ There is a second standard system database named <literal>template0</>.
+ This database contains the same data as the initial contents of
+ <literal>template1</>, that is, only the standard objects predefined by
+ your version of Postgres. <literal>template0</> should never be changed
+ after <literal>initdb</>. By instructing <command>CREATE DATABASE</> to
+ copy <literal>template0</> instead of <literal>template1</>, you can
+ create a <quote>virgin</> user database that contains none of the
+ site-local additions in <literal>template1</>. This is particularly
+ handy when restoring a <literal>pg_dump</> dump: the dump script should
+ be restored in a virgin database to ensure that one recreates the
+ correct contents of the dumped database, without any conflicts with
+ additions that may now be present in <literal>template1</>.
+ </para>
+
+ <para>
+ It is possible to create additional template databases, and indeed
+ one might copy any database in an installation by specifying its name
+ as the template for <command>CREATE DATABASE</>. It is important to
+ understand, however, that this is not (yet) intended as
+ a general-purpose COPY DATABASE facility. In particular, it is
+ essential that the source database be idle (no data-altering transactions
+ in progress)
+ for the duration of the copying operation. <command>CREATE DATABASE</>
+ will check
+ that no backend processes (other than itself) are connected to
+ the source database at the start of the operation, but this does not
+ guarantee that changes cannot be made while the copy proceeds, which
+ would result in an inconsistent copied database. Therefore,
+ we recommend that databases used as templates be treated as read-only.
+ </para>
+
+ <para>
+ Two useful flags exist in <literal>pg_database</literal> for each
+ database: <literal>datistemplate</literal> and
+ <literal>datallowconn</literal>. <literal>datistemplate</literal>
+ may be set to indicate that a database is intended as a template for
+ <command>CREATE DATABASE</>. If this flag is set, the database may be
+ cloned by
+ any user with CREATEDB privileges; if it is not set, only superusers
+ and the owner of the database may clone it.
+ If <literal>datallowconn</literal> is false, then no new connections
+ to that database will be allowed (but existing sessions are not killed
+ simply by setting the flag false). The <literal>template0</literal>
+ database is normally marked <literal>datallowconn</literal> =
+ <literal>false</> to prevent modification of it.
+ Both <literal>template0</literal> and <literal>template1</literal>
+ should always be marked with <literal>datistemplate</literal> =
+ <literal>true</>.
+ </para>
+
+ <para>
+ After preparing a template database, or making any changes to one,
+ it is a good idea to perform
+ <command>VACUUM FREEZE</> or <command>VACUUM FULL FREEZE</> in that
+ database. If this is done when there are no other open transactions
+ in the same database, then it is guaranteed that all tuples in the
+ database are <quote>frozen</> and will not be subject to transaction
+ ID wraparound problems. This is particularly important for a database
+ that will have <literal>datallowconn</literal> set to false, since it
+ will be impossible to do routine maintenance <command>VACUUM</>s on
+ such a database.
+ See <xref linkend="vacuum-for-wraparound"> for more information.
+ </para>
+
+ <note>
+ <para>
+ <literal>template1</> and <literal>template0</> do not have any special
+ status beyond the fact that the name <literal>template1</> is the default
+ source database name for <command>CREATE DATABASE</> and the default
+ database-to-connect-to for various scripts such as <literal>createdb</>.
+ For example, one could drop <literal>template1</> and recreate it from
+ <literal>template0</> without any ill effects. This course of action
+ might be advisable if one has carelessly added a bunch of junk in
+ <literal>template1</>.
+ </para>
+ </note>
+
+ </sect2>
+
+ <sect2 id="manage-ag-alternate-locs">
<title>Alternative Locations</title>
<para>
It is possible to create a database in a location other than the
- default. Remember that all database access occurs through the
- database server backend, so that any location specified must be
- accessible by the backend.
+ default location for the installation. Remember that all database access
+ occurs through the
+ database server, so any location specified must be
+ accessible by the server.
</para>
<para>
Alternative database locations are referenced by an environment
variable which gives the absolute path to the intended storage
- location. This environment variable must have been defined before
- the backend was started. Any valid environment variable name may
+ location. This environment variable must be present in the server's
+ environment, so it must have been defined before the server
+ was started. (Thus, the set of available alternative locations is
+ under the site administrator's control; ordinary users can't
+ change it.) Any valid environment variable name may
be used to reference an alternative location, although using
variable names with a prefix of <literal>PGDATA</> is recommended
to avoid confusion and conflict with other variables.
<para>
<indexterm><primary>initlocation</></>
To create a data storage area in <envar>PGDATA2</>, ensure that
- <filename>/home/postgres</filename> already exists and is writable
+ the containing directory (here, <filename>/home/postgres</filename>)
+ already exists and is writable
by the user account that runs the server (see <xref
linkend="postgres-user">). Then from the command line, type
<informalexample>
</para>
<para>
- To create a database at the new location, use the command
+ To create a database within the new location, use the command
<synopsis>
CREATE DATABASE <replaceable>name</> WITH LOCATION = '<replaceable>location</>'
</synopsis>
</para>
<para>
- Database created at alternative locations using this method can be
+ Databases created in alternative locations can be
accessed and dropped like any other database.
</para>
</sect2>
</sect1>
- <sect1 id="manage-ag-accessdb">
- <title>Accessing a Database</title>
-
- <para>
- Once you have constructed a database, you can access it by:
-
- <itemizedlist spacing="compact" mark="bullet">
- <listitem>
- <para>
- running the <productname>Postgres</productname> terminal monitor program
- (<application>psql</application>) which allows you to interactively
- enter, edit, and execute <acronym>SQL</acronym> commands.
- </para>
- </listitem>
-
- <listitem>
- <para>
- writing a C program using the <literal>libpq</literal> subroutine
- library. This allows you to submit <acronym>SQL</acronym> commands
- from C and get answers and status messages back to
- your program. This interface is discussed further
- in the <citetitle>PostgreSQL Programmer's Guide</citetitle>.
- </para>
- </listitem>
- </itemizedlist>
-
- You might want to start up <application>psql</application>,
- to try out the examples in this manual. It can be activated for the
- <replaceable class="parameter">dbname</replaceable> database by typing the command:
-
-<programlisting>
-psql <replaceable class="parameter">dbname</replaceable>
-</programlisting>
-
- You will be greeted with the following message:
-
-<programlisting>
-Welcome to psql, the PostgreSQL interactive terminal.
-
-Type: \copyright for distribution terms
- \h for help with SQL commands
- \? for help on internal slash commands
- \g or terminate with semicolon to execute query
- \q to quit
-
-<replaceable>dbname</replaceable>=>
-</programlisting>
- </para>
-
- <para>
- This prompt indicates that the terminal monitor is listening
- to you and that you can type <acronym>SQL</acronym> queries into a
- workspace maintained by the terminal monitor.
- The <application>psql</application> program responds to escape
- codes that begin
- with the backslash character, <literal>\</literal>. For example, you
- can get help on the syntax of various
- <productname>Postgres</productname> <acronym>SQL</acronym> commands by typing:
-
- <programlisting>
-<replaceable>dbname</replaceable>=> \h
- </programlisting>
-
- Once you have finished entering your queries into the
- workspace, you can pass the contents of the workspace
- to the <productname>Postgres</productname> server by typing:
-
- <programlisting>
-<replaceable>dbname</replaceable>=> \g
- </programlisting>
-
- This tells the server to process the query. If you
- terminate your query with a semicolon, the backslash-g is not
- necessary. <application>psql</application> will automatically
- process semicolon terminated queries.
- To read queries from a file, instead of
- entering them interactively, type:
-
- <programlisting>
-<replaceable>dbname</replaceable>=> \i <replaceable class="parameter">filename</replaceable>
- </programlisting>
-
- To get out of <application>psql</application> and return to Unix, type
-
- <programlisting>
-<replaceable>dbname</replaceable>=> \q
- </programlisting>
-
- and <application>psql</application> will quit and return
- you to your command shell. (For more escape codes, type
- backslash-h at the monitor prompt.)
- White space (i.e., spaces, tabs and newlines) may be
- used freely in <acronym>SQL</acronym> queries.
- Single-line comments are denoted by two dashes
- ("<literal>--</literal>"). Everything after the dashes up to the end of the
- line is ignored. Multiple-line comments, and comments within a line,
- are denoted by <literal>/* ... */</literal>, a convention borrowed
- from <productname>Ingres</productname>.
- </para>
- </sect1>
-
<sect1 id="manage-ag-dropdb">
<title>Destroying a Database</title>
<synopsis>
DROP DATABASE <replaceable>name</>
</synopsis>
- Only the owner of the database (i.e., the user that created it) can
- drop databases. Dropping a databases removes all objects that were
+ Only the owner of the database (i.e., the user that created it), or
+ a superuser, can drop a database. Dropping a database removes all objects
+ that were
contained within the database. The destruction of a database cannot
be undone.
</para>
<para>
You cannot execute the <command>DROP DATABASE</command> command
while connected to the victim database. You can, however, be
- connected to any other database, including the template1 database,
- which would be the only option for dropping the last database of a
+ connected to any other database, including the <literal>template1</>
+ database,
+ which would be the only option for dropping the last user database of a
given cluster.
</para>
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<Chapter Id="manage">
designated the <FirstTerm>database administrator</FirstTerm>.
This assignment of responsibilities occurs when a database is created.
A user may be assigned explicit privileges to create databases and/or to create new users.
- A user assigned both privileges can perform most administrative task
+ A user assigned both privileges can perform most administrative tasks
within <ProductName>Postgres</ProductName>, but will
not by default have the same operating system privileges as the site administrator.
</Para>
<Para>
- The Database Administrator's Guide covers these topics in more detail.
+ The <citetitle>Administrator's Guide</> covers these topics in
+ more detail.
</Para>
<Sect1 id="db-creation">
<Title>Database Creation</Title>
<Para>
- Databases are created by the <Command>CREATE DATABASE</Command> issued from
- within <ProductName>Postgres</ProductName>. <Application>createdb</Application> is a command-line
- utility provided to give the same functionality from outside <ProductName>Postgres</ProductName>.
+ Databases are created by the <Command>CREATE DATABASE</Command>
+ command issued from within
+ <ProductName>Postgres</ProductName>. <Application>createdb</Application>
+ is a shell script provided to give the same functionality from the
+ Unix command line.
</Para>
<Para>
</Para>
<Para>
- <ProductName>Postgres</ProductName> allows you to create any number of databases
- at a given site and you automatically become the
+ You automatically become the
database administrator of the database you just created.
Database names must have an alphabetic first
- character and are limited to 32 characters in length.
+ character and are limited to 31 characters in length.
+ <ProductName>Postgres</ProductName> allows you to create any number of
+ databases at a given site.
</Para>
- </Sect1>
-
- <Sect1 id="altern-locations">
- <Title>Alternate Database Locations</Title>
-
- <Para>
- It is possible to create a database in a location other than the default
- location for the installation. Remember that all database access actually
- occurs through the database backend, so that any location specified must
- be accessible by the backend.
- </Para>
-
- <Para>
- Alternate database locations are created and referenced by an environment variable
- which gives the absolute path to the intended storage location.
- This environment variable must have been defined before the postmaster was started
- and the location it points to must be writable by the administrator account.
- Consult with the site administrator
- regarding preconfigured alternative database locations.
- Any valid environment variable name may be used to reference an alternate location,
- although using variable names with a prefix of <envar>PGDATA</envar> is recommended
- to avoid confusion
- and conflict with other variables.
- </Para>
-
- <Note>
- <Para>
- In previous versions of <ProductName>Postgres</ProductName>,
- it was also permissible to use an absolute path name to specify
- an alternate storage location.
- Although the environment variable style of specification
- is to be preferred since it allows the site administrator more flexibility in
- managing disk storage, it is also possible to use an absolute path
- to specify an alternate location.
- The administrator's guide discusses how to enable this feature.
- </Para>
- </Note>
-
- <Para>
- For security and integrity reasons,
- any path or environment variable specified has some
- additional path fields appended.
- Alternate database locations must be prepared by running
- <Application>initlocation</Application>.
- </Para>
-
- <Para>
- To create a data storage area using the environment variable
- <envar>PGDATA2</envar> (for this example set to <filename>/alt/postgres</filename>),
- ensure that <FileName>/alt/postgres</FileName> already exists and is writable by
- the Postgres administrator account.
- Then, from the command line, type
-<ProgramListing>
-% initlocation PGDATA2
-The location will be initialized with username "postgres".
-This user will own all the files and must also own the server process.
-
-Creating directory /alt/postgres/data
-Creating directory /alt/postgres/data/base
-
-initlocation is complete.
-You can now create a database using
- CREATE DATABASE <name> WITH LOCATION = 'PGDATA2'
-in SQL, or
- createdb <name> -D 'PGDATA2'
-from the shell.
-</ProgramListing>
- </Para>
-
- <Para>
- To create a database in the alternate storage area <envar>PGDATA2</envar>
- from the command line, use the following command:
-<ProgramListing>
-% createdb -D PGDATA2 mydb
-</ProgramListing>
-
- and to do the same from within <Application>psql</Application> type
-<ProgramListing>
-=> CREATE DATABASE mydb WITH LOCATION = 'PGDATA2';
-</ProgramListing>
- </Para>
-
- <Para>
- If you do not have the privileges required to create a database, you will see
- the following:
-<ProgramListing>
-ERROR: CREATE DATABASE: permission denied
-</ProgramListing>
- </Para>
-
- <Para>
- If the specified location does not exist or the database backend does not have
- permission to access it or to write to directories under it, you will see
- the following:
-<ProgramListing>
-ERROR: The database path '/no/where' is invalid. This may be due to a character that is not allowed or because the chosen path isn't permitted for databases.
-</ProgramListing>
- </Para>
+ <Para>
+ The <citetitle>Administrator's Guide</> discusses database creation
+ in more detail, including advanced options of the <command>CREATE
+ DATABASE</> command.
+ </Para>
</Sect1>
</ListItem>
<ListItem>
<Para>
- writing a C program using the <application>LIBPQ</application> subroutine
- library. This allows you to submit <Acronym>SQL</Acronym> commands
- from C and get answers and status messages back to
- your program. This interface is discussed further
- in <citetitle>The PostgreSQL Programmer's Guide</citetitle>.
+writing a C program using the <application>LIBPQ</application> subroutine
+library. This allows you to submit <Acronym>SQL</Acronym> commands
+from C and get answers and status messages back to
+your program. This interface is discussed further
+in <citetitle>The PostgreSQL Programmer's Guide</citetitle>.
+</Para>
+</ListItem>
+<ListItem>
+<Para>
+writing a program in other languages for which there are available interface
+libraries.
</Para>
</ListItem>
</ItemizedList>
This prompt indicates that <command>psql</command> is listening
to you and that you can type <Acronym>SQL</Acronym> queries into a
workspace maintained by the terminal monitor.
- The <Application>psql</Application> program responds to escape codes that begin
+The <Application>psql</Application> program itself responds to special
+commands that begin
with the backslash character, <literal>\</literal>. For example, you
can get help on the syntax of various
<ProductName>PostgreSQL</ProductName> <Acronym>SQL</Acronym> commands by typing:
<Para>
If you are the owner of the database
- <Database>mydb</Database>, you can destroy it using the following Unix command:
+ <Database>mydb</Database>, you can destroy it using the SQL command
+<ProgramListing>
+=> DROP DATABASE mydb;
+</ProgramListing>
+ or the Unix shell script
<ProgramListing>
% dropdb mydb
</ProgramListing>
<!--
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+$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_database.sgml,v 1.21 2001/11/18 00:38:00 tgl Exp $
Postgres documentation
-->
by writing <literal>TEMPLATE = template0</>, you can create a virgin
database containing only the standard objects predefined by your
version of Postgres. This is useful if you wish to avoid copying
- any installation-local objects that may have been added to template1.
+ any installation-local objects that may have been added to
+ <literal>template1</>.
</para>
<para>
-->
<para>
- Although it is possible to copy a database other than template1 by
- specifying its name as the template, this is not (yet) intended as
- a general-purpose COPY DATABASE facility. In particular, it is
- essential that the source database be idle (no data-altering transactions
- in progress)
- for the duration of the copying operation. CREATE DATABASE will check
- that no backend processes (other than itself) are connected to
- the source database at the start of the operation, but this does not
- guarantee that changes cannot be made while the copy proceeds. Therefore,
- we recommend that databases used as templates be treated as read-only.
- </para>
-
- <para>
- Two useful flags exist in <literal>pg_database</literal> for each
- database: <literal>datistemplate</literal> and
- <literal>datallowconn</literal>. <literal>datistemplate</literal>
- may be set to indicate that a database is intended as a template for
- CREATE DATABASE. If this flag is set, the database may be cloned by
- any user with CREATEDB privileges; if it is not set, only superusers
- and the owner of the database may clone it.
- If <literal>datallowconn</literal> is false, then no new connections
- to that database will be allowed (but existing sessions are not killed
- simply by setting the flag false). The <literal>template0</literal>
- database is normally marked this way to prevent modification of it.
- </para>
-
- <para>
- After preparing a template database, or making any changes to one,
- it is a good idea to perform
- <command>VACUUM FREEZE</> or <command>VACUUM FULL FREEZE</> in that
- database. If this is done when there are no other open transactions
- in the same database, then it is guaranteed that all tuples in the
- database are <quote>frozen</> and will not be subject to transaction
- ID wraparound problems. This is particularly important for a database
- that will have <literal>datallowconn</literal> set to false, since it
- will be impossible to do routine maintenance <command>VACUUM</>s on
- such a database.
- See the Administrator's Guide for more information.
+ Although it is possible to copy a database other than <literal>template1</>
+ by specifying its name as the template, this is not (yet) intended as
+ a general-purpose COPY DATABASE facility.
+ We recommend that databases used as templates be treated as read-only.
+ See the <citetitle>Administrator's Guide</> for more information.
</para>
</refsect2>
</refsect1>
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