Convert more charset/locale documentation to DocBook

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

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<chapter id="charset">
 <title>Localization</>

 <abstract>
  <para>
   Describes the available localization features from the point of
   view of the administrator.
  </para>
 </abstract>

  <para>
   <productname>Postgres</productname> supports localization with
   three approaches:

   <itemizedlist>
    <listitem>
     <para>
      Using the locale features of the operating system to provide
      locale-specific collation order, number formatting, and other
      aspects.
     </para>
    </listitem>

    <listitem>
     <para>
      Using explicit multiple-byte character sets defined in the
      <productname>Postgres</productname> server to support languages
      that require more characters than will fit into a single byte,
      and to provide character set recoding between client and server.
      The number of supported character sets is fixed at the time the
      server is compiled, and internal operations such as string
      comparisons require expansion of each character into a 32-bit
      word.
     </para>
    </listitem>

    <listitem>
     <para>
      Single byte character recoding provides a more light-weight
      solution for users of multiple, yet single-byte character sets.
     </para>
    </listitem>
   </itemizedlist>
  </para>


 <sect1 id="locale">
  <title>Locale Support</title>
  
  <para>
   <firstterm>Locale</> support refers to an application respecting
   cultural preferences regarding alphabets, sorting, number
   formatting, etc.  <productname>PostgreSQL</> uses the standard ISO
   C and POSIX-like locale facilities provided by the server operating
   system.  For additional information refer the documentation of your
   system.
  </para>

  <sect2>
   <title>Overview</>

  <para>
    Locale support is not build into <productname>PostgreSQL</> by
    default; to enable it, supply the <option>--enable-locale</> option
    to the <filename>configure</> script:
<informalexample>
<screen>
<prompt>$ </><userinput>./configure --enable-locale</>
</screen>
</informalexample>
    Locale support only affects the server; all clients are compatible
    with servers with or without locale support.
   </para>

   <para>
    The information about which particular cultural rules to use is
    determined by standard environment variables.  If you are getting
    localized behavior from other programs you probably have them set
    up already.  The simplest way to set the localization information
    is the <envar>LANG</> variable, for example:
<programlisting>
export LANG=sv_SE
</programlisting>
    This sets the locale to Swedish (<literal>sv</>) as spoken in
    Sweden (<literal>SE</>).  Other possibilities might be
    <literal>en_US</> (U.S. English) and <literal>fr_CA</> (Canada,
    French).  If more than one character set can be useful for a locale
    then the specifications look like this:
    <literal>cs_CZ.ISO8859-2</>. What locales are available under what
    names on your system depends on what was provided by the operating
    system vendor and what was installed.
   </para>

   <para>
    Occasionally it is useful to mix rules from several locales, e.g.,
    use U.S. rules but Spanish messages.  To do that a set of
    environment variables exist that override the default of
    <envar>LANG</> for a particular category:

    <informaltable>
     <tgroup cols="2">
      <tbody>
       <row>
        <entry>LC_COLLATE</>
        <entry>String sort order</>
       </row>
       <row>
        <entry>LC_CTYPE</>
        <entry>Character classification (What is a letter? What is the upper-case equivalent of this letter?)</>
       </row>
       <row>
        <entry>LC_MESSAGES</>
        <entry>Language of messages</>
       </row>
       <row>
        <entry>LC_MONETARY</>
        <entry>Formatting of currency amounts</>
       </row>
       <row>
        <entry>LC_NUMERIC</>
        <entry>Formatting of numbers</>
       </row>
       <row>
        <entry>LC_TIME</>
        <entry>Formatting of dates and times</>
       </row>
      </tbody>
     </tgroup>
    </informaltable>

    <envar>LC_MESSAGES</> only affects the messages that come from the
    operating system, not <productname>PostgreSQL</>.
   </para>

   <para>
    If you want the system to behave as if it had no locale support,
    use the special locale <literal>C</> or <literal>POSIX</>, or
    simply unset all locale related variables.
   </para>

   <para>
    Once you have chosen a set of localization rules this way you must
    keep them fixed for any particular database cluster.  That means
    that the locales that were active when you ran <filename>initdb</>
    must be kept the same when you start the postmaster.  Otherwise,
    the changed sort order can corrupt indexes or make your data
    disappear mysteriously.  It is currently not possible to change the
    locales after database initialization or to use more than one set
    of locales for a given database cluster.
   </para>
  </sect2>

  <sect2>
   <title>Benefits</>

   <para>
    Locale support influences in particular the following features:

    <itemizedlist>
     <listitem>
      <para>
       Sort order in <command>ORDER BY</> queries.
      </para>
     </listitem>

     <listitem>
      <para>
       The <function>to_char</> family of functions
      </para>
     </listitem>

     <listitem>
      <para>
       The <literal>LIKE</> and <literal>~</> operators for pattern
       matching
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    The only severe drawback of using the locale support in
    <productname>PostgreSQL</> is its speed.  So use locale only if you
    actually need it.
   </para>
  </sect2>

  <sect2>
   <title>Problems</>

   <para>
    If locale support doesn't work in spite of the explanation above,
    check that the locale support in your operating system is okay.
    To check whether a given locale is installed and functional you
    can use <application>Perl</>, for example.  Perl has also support
    for locales and if a locale is broken <command>perl -v</> will
    complain something like this:
<screen>
<prompt>$</> <userinput>export LC_CTYPE='not_exist'</>
<prompt>$</> <userinput>perl -v</>
<computeroutput>
perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
LC_ALL = (unset),
LC_CTYPE = "not_exist",
LANG = (unset)
are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
</computeroutput>
</screen>
   </para>

   <para>
    Check that your locale files are in the right location.  Possible
    locations include: <filename>/usr/lib/locale</filename> (Linux,
    Solaris), <filename>/usr/share/locale</filename> (Linux),
    <filename>/usr/lib/nls/loc</filename> (DUX 4.0).  Check the locale
    man page of your system if you are not sure.
   </para>

   <para>
    The directory <filename>src/test/locale</> contains a test suite
    for <productname>PostgreSQL</>'s locale support.
   </para>
  </sect2>
 </sect1>


  <sect1 id="multibyte">
   <title>Multibyte Support</title>

   <note>
    <title>Author</title>

    <para>
     <ulink url="mailto:ishii@postgresql.org">Tatsuo Ishii</ulink>,
     last updated 2000-03-22.
     Check <ulink
      url="http://www.sra.co.jp/people/t-ishii/PostgreSQL/">Tatsuo's
      web site</ulink> for more information.
    </para>
   </note>

   <para>
    Multibyte (<acronym>MB</acronym>) support is intended to allow
    <productname>Postgres</productname> to handle
    multiple-byte character sets such as EUC (Extended Unix Code), Unicode and
    Mule internal code. With <acronym>MB</acronym> enabled you can use multi-byte
    character sets in regular expressions (regexp), LIKE, and some
    other functions. The default
    encoding system is selected while initializing your
    <productname>Postgres</productname> installation using
    <application>initdb</application>. Note that this can be
    overridden when you create a database using
    <application>createdb</application> or by using the SQL command
    CREATE DATABASE. So you can have multiple databases each with
    a different encoding system.
   </para>

   <para>
    <acronym>MB</acronym> also fixes some problems concerning 8-bit single byte
    character sets including ISO8859. (I would not say all of problems
    have been fixed. I just confirmed that the regression test ran fine
    and a few French characters could be used with the patch. Please let
    me know if you find any problem while using 8-bit characters.)
   </para>

   <sect2>
    <title>Enabling MB</title>

    <para>
     Run configure with a multibyte option:

     <programlisting>
% ./configure --enable-multibyte[=<replaceable>encoding_system</replaceable>]
     </programlisting>

     where <replaceable>encoding_system</replaceable> can be one of the
     values in the following table:

     <table tocentry="1">
      <title><productname>Postgres</productname> Character Set Encodings</title>
      <titleabbrev>Encodings</titleabbrev>
      <tgroup cols="2">
       <thead>
        <row>
         <entry>Encoding</entry>
         <entry>Description</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry>SQL_ASCII</entry>
         <entry>ASCII</entry>
        </row>
        <row>
         <entry>EUC_JP</entry>
         <entry>Japanese EUC</entry>
        </row>
        <row>
         <entry>EUC_CN</entry>
         <entry>Chinese EUC</entry>
        </row>
        <row>
         <entry>EUC_KR</entry>
         <entry>Korean EUC</entry>
        </row>
        <row>
         <entry>EUC_TW</entry>
         <entry>Taiwan EUC</entry>
        </row>
        <row>
         <entry>UNICODE</entry>
         <entry>Unicode(UTF-8)</entry>
        </row>
        <row>
         <entry>MULE_INTERNAL</entry>
         <entry>Mule internal</entry>
        </row>
        <row>
         <entry>LATIN1</entry>
         <entry>ISO 8859-1 English and some European languages</entry>
        </row>
        <row>
         <entry>LATIN2</entry>
         <entry>ISO 8859-2 English and some European languages</entry>
        </row>
        <row>
         <entry>LATIN3</entry>
         <entry>ISO 8859-3 English and some European languages</entry>
        </row>
        <row>
         <entry>LATIN4</entry>
         <entry>ISO 8859-4 English and some European languages</entry>
        </row>
        <row>
         <entry>LATIN5</entry>
         <entry>ISO 8859-5 English and some European languages</entry>
        </row>
        <row>
         <entry>KOI8</entry>
         <entry>KOI8-R</entry>
        </row>
        <row>
         <entry>WIN</entry>
         <entry>Windows CP1251</entry>
        </row>
        <row>
         <entry>ALT</entry>
         <entry>Windows CP866</entry>
        </row>
       </tbody>
      </tgroup>
     </table>
    </para>

    <para>
     Here is an example of configuring
     <productname>Postgres</productname> to use a Japanese encoding by
     default:

     <programlisting>
% ./configure --enable-multibyte=EUC_JP
     </programlisting>
    </para>

    <para>
     If the encoding system is omitted (./configure --enable-multibyte),
     SQL_ASCII is assumed.
    </para>
   </sect2>

   <sect2>
    <title>Setting the Encoding</title>

    <para>
     <application>initdb</application> defines the default encoding
     for a <productname>Postgres</productname> installation. For example:

     <programlisting>
% initdb -E EUC_JP
     </programlisting>

     sets the default encoding to EUC_JP(Extended Unix Code for Japanese).
     Note that you can use "--encoding" instead of "-E" if you prefer
     to type longer option strings. 
     If no -E or --encoding option is given, the encoding
     specified at the compile time is used.
    </para>

    <para>
     You can create a database with a different encoding:

     <programlisting>
% createdb -E EUC_KR korean
     </programlisting>

     will create a database named "korean" with EUC_KR encoding. The
     another way to accomplish this is to use a SQL command:

     <programlisting>
CREATE DATABASE korean WITH ENCODING = 'EUC_KR';
     </programlisting>

     The encoding for a database is represented as an
     <firstterm>encoding column</firstterm>  in the
     <literal>pg_database</literal> system catalog.
     You can see that by using -l or \l of psql
     command.

     <programlisting>
$ psql -l
            List of databases
   Database    |  Owner  |   Encoding    
---------------+---------+---------------
 euc_cn        | t-ishii | EUC_CN
 euc_jp        | t-ishii | EUC_JP
 euc_kr        | t-ishii | EUC_KR
 euc_tw        | t-ishii | EUC_TW
 mule_internal | t-ishii | MULE_INTERNAL
 regression    | t-ishii | SQL_ASCII
 template1     | t-ishii | EUC_JP
 test          | t-ishii | EUC_JP
 unicode       | t-ishii | UNICODE
(9 rows)
     </programlisting>
    </para>
   </sect2>

   <sect2>
    <title>Automatic encoding translation between backend and
     frontend</title>

    <para>
     <productname>Postgres</productname> supports an automatic
     encoding translation between backend
     and frontend for some encodings.

     <table tocentry="1">
      <title><productname>Postgres</productname> Client/Server Character Set Encodings</title>
      <titleabbrev>Communication Encodings</titleabbrev>
      <tgroup cols="2">
       <thead>
        <row>
         <entry>Server Encoding</entry>
         <entry>Available Client Encodings</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry>EUC_JP</entry>
         <entry>EUC_JP, SJIS</entry>
        </row>
        <row>
         <entry>EUC_TW</entry>
         <entry>EUC_TW, BIG5</entry>
        </row>
        <row>  
         <entry>LATIN2</entry>
         <entry>LATIN2, WIN1250</entry>
        </row>
        <row>
         <entry>LATIN5</entry>
         <entry>LATIN5, WIN, ALT</entry>
        </row>
        <row>
         <entry>MULE_INTERNAL</entry>
         <entry>EUC_JP, SJIS, EUC_KR, EUC_CN, 
          EUC_TW, BIG5, LATIN1 to LATIN5, 
          WIN, ALT, WIN1250</entry>
        </row>
       </tbody>
      </tgroup>
     </table>
    </para>

    <para>
     To enable the automatic encoding translation, you have to tell
     <productname>Postgres</productname> the encoding you would like
     to use in frontend. There are
     several ways to accomplish this.

     <itemizedlist>
      <listitem>
       <para>
        Using the <command>\encoding</command> command in
        <application>psql</application>.
        <command>\encoding</command> allows you to change frontend
        encoding on the fly. For
        example, to change the encoding to SJIS, type:

        <programlisting>
\encoding SJIS
        </programlisting>
       </para>
      </listitem>

      <listitem>
       <para>
        Using libpq functions.
        <command>\encoding</command> actually calls
        PQsetClientEncoding() for its purpose.

        <programlisting>
int PQsetClientEncoding(PGconn *<replaceable>conn</replaceable>, const char *<replaceable>encoding</replaceable>)
        </programlisting>

        where <replaceable>conn</replaceable> is a connection to the backend,
        and <replaceable>encoding</replaceable> is an encoding you
        want to use. If it successfully sets the encoding, it returns 0,
        otherwise -1. The current encoding for this connection can be shown by
        using:

        <programlisting>
int PQclientEncoding(const PGconn *<replaceable>conn</replaceable>)
        </programlisting>

        Note that it returns the "encoding id," not the encoding symbol string
        such as "EUC_JP." To convert an encoding id to an encoding symbol, you
        can use:

        <programlisting>
char *pg_encoding_to_char(int <replaceable>encoding_id</replaceable>)
        </programlisting>
       </para>
      </listitem>

      <listitem>
       <para>
        Using <envar>PGCLIENTENCODING</envar>.

        If an environment variable <envar>PGCLIENTENCODING</envar> is defined in the
        frontend, an automatic encoding translation is done by the backend.
       </para>
      </listitem>

      <listitem>
       <para>
        Using <command>SET CLIENT_ENCODING TO</command>.

        Setting the frontend side encoding can be done a SQL command:

        <programlisting>
SET CLIENT_ENCODING TO 'encoding';
        </programlisting>

        Also you can use SQL92 syntax "SET NAMES" for this purpose:

        <programlisting>
SET NAMES 'encoding';
        </programlisting>

        To query the current the frontend encoding:

        <programlisting>
SHOW CLIENT_ENCODING;
        </programlisting>

        To return to the default encoding:

        <programlisting>
RESET CLIENT_ENCODING;
        </programlisting>
       </para>
      </listitem>
     </itemizedlist>
    </para>
   </sect2>

   <sect2>
    <title>About Unicode</title>

    <para>
     An automatic encoding translation between Unicode and other
     encodings is not yet supported.
    </para>
   </sect2>

   <sect2>
    <title>What happens if the translation is not possible?</title>

    <para>
     Suppose you choose EUC_JP for the backend, LATIN1 for the frontend,
     then some Japanese characters could not be translated into LATIN1. In
     this case, a letter cannot be represented in the LATIN1 character set,
     would be transformed as:

     <programlisting>
(HEXA DECIMAL)
     </programlisting>
    </para>
   </sect2>

   <sect2>
    <title>References</title>

    <para>
     These are good sources to start learning various kind of encoding
     systems.

     <itemizedlist>
      <listitem>
       <para>
        <ulink url="ftp://ftp.ora.com/pub/examples/nutshell/ujip/doc/cjk.inf">
         ftp://ftp.ora.com/pub/examples/nutshell/ujip/doc/cjk.inf</ulink>
        Detailed explanations of EUC_JP, EUC_CN, EUC_KR, EUC_TW
        appear in section 3.2.
       </para>
      </listitem>

      <listitem>
       <para>
        Unicode: <ulink url="http://www.unicode.org/">http://www.unicode.org/</ulink>
        The homepage of UNICODE.
       </para>
      </listitem>

      <listitem>
       <para>
        <literal>RFC 2044</literal>
        UTF-8 is defined here.
       </para>
      </listitem>
     </itemizedlist>
    </para>
   </sect2>

   <sect2>
    <title>History</title>

    <para>
     <programlisting>
May 20, 2000
        * SJIS UDC (NEC selection IBM kanji) support contributed
          by Eiji Tokuya
        * Changes above will appear in 7.0.1

Mar 22, 2000
        * Add new libpq functions PQsetClientEncoding, PQclientEncoding
        * ./configure --with-mb=EUC_JP
          now deprecated. use 
          ./configure --enable-multibyte=EUC_JP
          instead
        * Add SQL_ASCII regression test case
        * Add SJIS User Defined Character (UDC) support
        * All of above will appear in 7.0

July 11, 1999
        * Add support for WIN1250 (Windows Czech) as a client encoding
          (contributed by Pavel Behal)
        * fix some compiler warnings (contributed by Tomoaki Nishiyama)

Mar 23, 1999
        * Add support for KOI8(KOI8-R), WIN(CP1251), ALT(CP866)
          (thanks Oleg Broytmann for testing)
        * Fix problem with MB and locale

Jan 26, 1999
        * Add support for Big5 for fronend encoding
          (you need to create a database with EUC_TW to use Big5)
        * Add regression test case for EUC_TW
          (contributed by <ulink url="mailto:jonahkuo@mail.ttn.com.tw">Jonah Kuo</ulink>)

Dec 15, 1998
        * Bugs related to SQL_ASCII support fixed

Nov 5, 1998
        * 6.4 release. In this version, pg_database has "encoding"
          column that represents the database encoding

Jul 22, 1998
        * determine encoding at initdb/createdb rather than compile time
        * support for PGCLIENTENCODING when issuing COPY command
        * support for SQL92 syntax "SET NAMES"
        * support for LATIN2-5
        * add UNICODE regression test case
        * new test suite for MB
        * clean up source files

Jun 5, 1998
        * add support for the encoding translation between the backend
          and the frontend
        * new command SET CLIENT_ENCODING etc. added
        * add support for LATIN1 character set
        * enhance 8 bit cleaness

April 21, 1998 some enhancements/fixes
        * character_length(), position(), substring() are now aware of 
          multi-byte characters
        * add octet_length()
        * add --with-mb option to configure
        * new regression tests for EUC_KR
          (contributed by <ulink url="mailto:hong@lunaris.hanmesoft.co.kr">Soonmyung. Hong</ulink>)
        * add some test cases to the EUC_JP regression test
        * fix problem in regress/regress.sh in case of System V
        * fix toupper(), tolower() to handle 8bit chars

Mar 25, 1998 MB PL2 is incorporated into PostgreSQL 6.3.1

Mar 10, 1998 PL2 released
        * add regression test for EUC_JP, EUC_CN and MULE_INTERNAL
        * add an English document (this file)
        * fix problems concerning 8-bit single byte characters

Mar 1, 1998 PL1 released
     </programlisting>
    </para>
   </sect2>

   <sect2>
    <title>WIN1250 on Windows/ODBC</title>

    <para>
<!--
[Here is a good documentation explaining how to use WIN1250 on
Windows/ODBC from Pavel Behal. Please note that Installation step 1)
is not necceary in 6.5.1 - Tatsuo]

Version: 0.91 for PgSQL 6.5
Author: Pavel Behal
Revised by: Tatsuo Ishii
Email: <ulink url="mailto:behal@opf.slu.cz">behal@opf.slu.cz</ulink>
Licence: The Same as PostgreSQL

Sorry for my Eglish and C code, I'm not native :-)

!!!!!!!!!!!!!!!!!!!!!!!!! NO WARRANTY !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
-->

     The WIN1250 character set on Windows client platforms can be used
     with <productname>Postgres</productname> with locale support
     enabled.
    </para>

    <para>
     The following should be kept in mind:

     <itemizedlist>
      <listitem>
       <para>
        Success depends on proper system locales. This has been tested
        with RH6.0 and Slackware 3.6, with cs_CZ.iso8859-2 locale.
       </para>
      </listitem>

      <listitem>
       <para>
        Never try to set the server multibyte database encoding to WIN1250.
        Always use LATIN2 instead since there is not a WIN1250 locale
        in Unix.
       </para>
      </listitem>

      <listitem>
       <para>
        WIN1250 encoding is useable only for M$W ODBC clients. The
        characters are recoded on the fly, to be displayed and stored
        back properly.
       </para>
      </listitem>
     </itemizedlist>
    </para>

    <para>
     When running, it is important to remember the following:

     <itemizedlist>
      <listitem>
       <para>
        This configuration reorders your sort order depending on your
        <envar>LC_<replaceable>x</replaceable></envar> settings. Don't be
        confused with the regression test results since they don't use
        locale.
       </para>
      </listitem>

      <listitem>
       <para>
        A locale such as "<literal>ch</literal>" is correctly sorted
        only if your system
        supports that locale; older systems may not do so but new ones
        (e.g. RH6.0) do.
       </para>
      </listitem>

      <listitem>
       <para>
        You have to insert money as '<literal>162,50</literal>' (note
        comma within the single-quotes).
       </para>
      </listitem>

      <listitem>
       <para>
        At the time of writing (early 1999), this configuration has
        not received extensive testing. Please let us know of any
        changes you had to make!
       </para>
      </listitem>
     </itemizedlist>
    </para>

    <procedure>
     <title>WIN1250 on Windows/ODBC</title>
     <step>
      <para>
       Change the three relevant files in the source directories.
      </para>
     </step>

     <step>
      <para>
       Compile <productname>Postgres</productname> with local enabled
       and the multibyte encoding set to <literal>LATIN2</literal>.
      </para>
     </step>

     <step>
      <para>
       Set up your instalation. Do not forget to create locale
       variables in your profile (environment). For example (this may
       not be correct for <emphasis>your</emphasis> environment):

       <programlisting>
LC_ALL=cs_CZ.ISO8859-2
LC_COLLATE=cs_CZ.ISO8859-2
LC_CTYPE=cs_CZ.ISO8859-2
LC_MONETARY=cs_CZ.ISO8859-2
LC_NUMERIC=cs_CZ.ISO8859-2
LC_TIME=cs_CZ.ISO8859-2
       </programlisting>
      </para>
     </step>

     <step>
      <para>
       You have to start the postmaster with locales set!
      </para>
     </step>

     <step>
      <para>
       Try it with Czech language, and have it sort on a query.
      </para>
     </step>

     <step>
      <para>
       Install ODBC driver for PgSQL on your M$ Windows machine.
      </para>
     </step>

     <step>
      <para>
       Setup properly your data source. Include this line in your ODBC
       configuration dialog in the field <literal>Connect Settings</literal>:

       <programlisting>
SET CLIENT_ENCODING = 'WIN1250';
       </programlisting>
      </para>
     </step>

     <step>
      <para>
       Now try it again, but in Windows with ODBC.
      </para>
     </step>
    </procedure>
   </sect2>
  </sect1>


 <sect1 id="recode">
  <title>Single-byte character set recoding</>
<!-- formerly in README.charsets, by Josef Balatka, <balatka@email.cz> -->

  <para>
   You can set up this feature with the <option>--enable-recode</> option
   to <filename>configure</>. This option was formerly described as
   <quote>Cyrillic recode support</> which doesn't express all its
   power. It can be used for <emphasis>any</> single-byte character
   set recoding.
  </para>

  <para>
   This method uses a file <filename>charset.conf</> file located in
   the database directory (<envar>PGDATA</>).  It's a typical
   configuration text file where spaces and newlines separate items
   and records and # specifies comments.  Three keywords with the
   following syntax are recognized here:
<synopsis>
BaseCharset      <replaceable>server_charset</>
RecodeTable      <replaceable>from_charset</> <replaceable>to_charset</> <replaceable>file_name</>
HostCharset      <replaceable>host_spec</>    <replaceable>host_charset</>
</synopsis>
  </para>

  <para>
   <token>BaseCharset</> defines the encoding of the database server.
   All character set names are only used for mapping inside of
   <filename>charset.conf</> so you can freely use typing-friendly
   names.
  </para>

  <para>
   <token>RecodeTable</> records specify translation tables between
   server and client.  The file name is relative to the
   <envar>PGDATA</> directory.  The table file format is very
   simple. There are no keywords and characters are represented by a
   pair of decimal or hexadecimal (0x prefixed) values on single
   lines:
<synopsis>
<replaceable>char_value</>   <replaceable>translated_char_value</>
</synopsis>
  </para>

  <para>
   <token>HostCharset</> records define the client character set by IP
   address. You can use a single IP address, an IP mask range starting
   from the given address or an IP interval (e.g., 127.0.0.1,
   192.168.1.100/24, 192.168.1.20-192.168.1.40).
  </para>

  <para>
   The <filename>charset.conf</> file is always processed up to the
   end, so you can easily specify exceptions from the previous
   rules. In the src/data you will find charset.conf example and a few
   recoding tables.
  </para>

  <para>
   As this solution is based on the client's IP address and character
   set mapping there are obviously some restrictions as well. You
   cannot use different encodings on the same host at the same
   time. It is also inconvenient when you boot your client hosts into
   more operating systems.  Nevertheless, when these restrictions are
   not limiting and you do not need multi-byte characters than it is a
   simple and effective solution.
  </para>
 </sect1>

</chapter>

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