charset.sgml

PostgreSQL7.4.6 for Linux

<!-- $Header: /cvsroot/pgsql/doc/src/sgml/charset.sgml,v 2.39.2.1 2003/11/04 09:45:29 petere Exp $ -->

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

 <para>
  This chapter describes the available localization features from the
  point of view of the administrator.
  <productname>PostgreSQL</productname> supports localization with
  two approaches:

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

    <listitem>
     <para>
      Providing a number of different character sets defined in the
      <productname>PostgreSQL</productname> server, including
      multiple-byte character sets, to support storing text in all
      kinds of languages, and providing character set translation between
      client and server.
     </para>
    </listitem>
   </itemizedlist>
  </para>


 <sect1 id="locale">
  <title>Locale Support</title>
  
  <indexterm zone="locale"><primary>locale</></>

  <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 <acronym>POSIX</acronym> locale facilities provided by the server operating
   system.  For additional information refer to the documentation of your
   system.
  </para>

  <sect2>
   <title>Overview</>

   <para>
    Locale support is automatically initialized when a database
    cluster is created using <command>initdb</command>.
    <command>initdb</command> will initialize the database cluster
    with the locale setting of its execution environment; so if your
    system is already set to use the locale that you want in your
    database cluster then there is nothing else you need to do.  If
    you want to use a different locale (or you are not sure which
    locale your system is set to), you can tell
    <command>initdb</command> exactly which locale you want with the
    option <option>--locale</option>.  For example:
<screen>
initdb --locale=sv_SE
</screen>
   </para>

   <para>
    This example 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 English collation rules but Spanish messages.  To support that, a
    set of locale subcategories exist that control only a certain
    aspect of the localization rules.

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

    The category names translate into names of
    <command>initdb</command> options to override the locale choice
    for a specific category.  For instance, to set the locale to
    French Canadian, but use U.S. rules for formatting currency, use
    <literal>initdb --locale=fr_CA --lc-monetary=en_US</literal>.
   </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</>.
   </para>

   <para>
    The nature of some locale categories is that their value has to be
    fixed for the lifetime of a database cluster.  That is, once
    <command>initdb</command> has run, you cannot change them anymore.
    <literal>LC_COLLATE</literal> and <literal>LC_CTYPE</literal> are
    those categories.  They affect the sort order of indexes, so they
    must be kept fixed, or indexes on text columns will become corrupt.
    <productname>PostgreSQL</productname> enforces this by recording
    the values of <envar>LC_COLLATE</> and <envar>LC_CTYPE</> that are
    seen by <command>initdb</>.  The server automatically adopts
    those two values when it is started.
   </para>

   <para>
    The other locale categories can be changed as desired whenever the
    server is running by setting the run-time configuration variables
    that have the same name as the locale categories (see <xref
    linkend="runtime-config"> for details).  The defaults that are
    chosen by <command>initdb</command> are actually only written into
    the configuration file <filename>postgresql.conf</filename> to
    serve as defaults when the server is started.  If you delete the
    assignments from <filename>postgresql.conf</filename> then the
    server will inherit the settings from the execution environment.
   </para>

   <para>
    Note that the locale behavior of the server is determined by the
    environment variables seen by the server, not by the environment
    of any client.  Therefore, be careful to configure the correct locale settings
    before starting the server.  A consequence of this is that if
    client and server are set up to different locales, messages may
    appear in different languages depending on where they originated.
   </para>

   <note>
    <para>
     When we speak of inheriting the locale from the execution
     environment, this means the following on most operating systems:
     For a given locale category, say the collation, the following
     environment variables are consulted in this order until one is
     found to be set: <envar>LC_ALL</envar>, <envar>LC_COLLATE</envar>
     (the variable corresponding to the respective category),
     <envar>LANG</envar>.  If none of these environment variables are
     set then the locale defaults to <literal>C</literal>.
    </para>

    <para>
     Some message localization libraries also look at the environment
     variable <envar>LANGUAGE</envar> which overrides all other locale
     settings for the purpose of setting the language of messages.  If
     in doubt, please refer to the documentation of your operating
     system, in particular the documentation about
     <application>gettext</>, for more information.
    </para>
   </note>

   <para>
    To enable messages translated to the user's preferred language,
    <acronym>NLS</acronym> must have been enabled at build time.  This
    choice is independent of the other locale support.
   </para>
  </sect2>

  <sect2>
   <title>Benefits</>

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

    <itemizedlist>
     <listitem>
      <para>
       Sort order in queries using <command>ORDER BY</>
       <indexterm><primary>ORDER BY</><secondary>and locales</></indexterm>
      </para>
     </listitem>

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

   <para>
    The only severe drawback of using the locale support in
    <productname>PostgreSQL</> is its speed.  So use locales only if
    you actually need them.
   </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
    correctly configured.  To check what locales are installed on your
    system, you may use the command <literal>locale -a</literal> if
    your operating system provides it.
   </para>

   <para>
    Check that <productname>PostgreSQL</> is actually using the locale
    that you think it is.  <envar>LC_COLLATE</> and <envar>LC_CTYPE</>
    settings are determined at <command>initdb</> time and cannot be
    changed without repeating <command>initdb</>.  Other locale
    settings including <envar>LC_MESSAGES</> and <envar>LC_MONETARY</>
    are initially determined by the environment the server is started
    in.  You can check the <envar>LC_COLLATE</> and <envar>LC_CTYPE</>
    settings of a database with the utility program
    <command>pg_controldata</>.
   </para>

   <para>
    The directory <filename>src/test/locale</> in the source
    distribution contains a test suite for
    <productname>PostgreSQL</>'s locale support.
   </para>

   <para>
    Client applications that handle server-side errors by parsing the
    text of the error message will obviously have problems when the
    server's messages are in a different language.  Authors of such
    applications are advised to make use of the error code scheme
    instead.
   </para>

   <para>
    Maintaining catalogs of message translations requires the on-going
    efforts of many volunteers that want to see
    <productname>PostgreSQL</> speak their preferred language well.
    If messages in your language is currently not available or fully
    translated, your assistance would be appreciated.  If you want to
    help, refer to the <xref linkend="nls"> or write to the developers'
    mailing list.
   </para>
  </sect2>
 </sect1>


 <sect1 id="multibyte">
  <title>Character Set Support</title>

  <indexterm zone="multibyte"><primary>character set</></>

  <para>
   The character set support in <productname>PostgreSQL</productname>
   allows you to store text in a variety of character sets, including
   single-byte character sets such as the ISO 8859 series and
   multiple-byte character sets such as <acronym>EUC</> (Extended Unix
   Code), Unicode, and Mule internal code.  All character sets can be
   used transparently throughout the server.  (If you use extension
   functions from other sources, it depends on whether they wrote
   their code correctly.)  The default character set is selected while
   initializing your <productname>PostgreSQL</productname> database
   cluster using <command>initdb</>.  It can be overridden when you
   create a database using <command>createdb</command> or by using the
   SQL command <command>CREATE DATABASE</>. So you can have multiple
   databases each with a different character set.
  </para>

   <sect2>
    <title>Supported Character Sets</title>

    <para>
     <xref linkend="charset-table"> shows the character sets available
     for use in the server.
    </para>

     <table id="charset-table">
      <title>Server Character Sets</title>
      <tgroup cols="2">
       <thead>
	<row>
	 <entry>Name</entry>
	 <entry>Description</entry>
	</row>
       </thead>
       <tbody>
	<row>
	 <entry><literal>SQL_ASCII</literal></entry>
	 <entry><acronym>ASCII</acronym></entry>
	</row>
	<row>
	 <entry><literal>EUC_JP</literal></entry>
	 <entry>Japanese <acronym>EUC</></entry>
	</row>
	<row>
	 <entry><literal>EUC_CN</literal></entry>
	 <entry>Chinese <acronym>EUC</></entry>
	</row>
	<row>
	 <entry><literal>EUC_KR</literal></entry>
	 <entry>Korean <acronym>EUC</></entry>
	</row>
	<row>
	 <entry><literal>JOHAB</literal></entry>
	 <entry>Korean <acronym>EUC</> (Hangle base)</entry>
	</row>
	<row>
	 <entry><literal>EUC_TW</literal></entry>
	 <entry>Taiwan <acronym>EUC</acronym></entry>
	</row>
	<row>
	 <entry><literal>UNICODE</literal></entry>
	 <entry>Unicode (<acronym>UTF</acronym>-8)</entry>
	</row>
	<row>
	 <entry><literal>MULE_INTERNAL</literal></entry>
	 <entry>Mule internal code</entry>
	</row>
	<row>
	 <entry><literal>LATIN1</literal></entry>
	 <entry>ISO 8859-1/<acronym>ECMA</> 94 (Latin alphabet no.1)</entry>
	</row>
	<row>
	 <entry><literal>LATIN2</literal></entry>
	 <entry>ISO 8859-2/<acronym>ECMA</> 94 (Latin alphabet no.2)</entry>
	</row>
	<row>
	 <entry><literal>LATIN3</literal></entry>
	 <entry>ISO 8859-3/<acronym>ECMA</> 94 (Latin alphabet no.3)</entry>
	</row>
	<row>
	 <entry><literal>LATIN4</literal></entry>
	 <entry>ISO 8859-4/<acronym>ECMA</> 94 (Latin alphabet no.4)</entry>
	</row>
	<row>
	 <entry><literal>LATIN5</literal></entry>
	 <entry>ISO 8859-9/<acronym>ECMA</> 128 (Latin alphabet no.5)</entry>
	</row>
	<row>
	 <entry><literal>LATIN6</literal></entry>
	 <entry>ISO 8859-10/<acronym>ECMA</> 144 (Latin alphabet no.6)</entry>
	</row>
	<row>
	 <entry><literal>LATIN7</literal></entry>
	 <entry>ISO 8859-13 (Latin alphabet no.7)</entry>
	</row>
	<row>
	 <entry><literal>LATIN8</literal></entry>
	 <entry>ISO 8859-14 (Latin alphabet no.8)</entry>
	</row>
	<row>
	 <entry><literal>LATIN9</literal></entry>
	 <entry>ISO 8859-15 (Latin alphabet no.9)</entry>
	</row>
	<row>
	 <entry><literal>LATIN10</literal></entry>
	 <entry>ISO 8859-16/<acronym>ASRO</> SR 14111 (Latin alphabet no.10)</entry>
	</row>
	<row>
	 <entry><literal>ISO_8859_5</literal></entry>
	 <entry>ISO 8859-5/<acronym>ECMA</> 113 (Latin/Cyrillic)</entry>
	</row>
	<row>
	 <entry><literal>ISO_8859_6</literal></entry>
	 <entry>ISO 8859-6/<acronym>ECMA</> 114 (Latin/Arabic)</entry>
	</row>
	<row>
	 <entry><literal>ISO_8859_7</literal></entry>
	 <entry>ISO 8859-7/<acronym>ECMA</> 118 (Latin/Greek)</entry>
	</row>
	<row>
	 <entry><literal>ISO_8859_8</literal></entry>
	 <entry>ISO 8859-8/<acronym>ECMA</> 121 (Latin/Hebrew)</entry>
	</row>
	<row>
	 <entry><literal>KOI8</literal></entry>
	 <entry><acronym>KOI</acronym>8-R(U)</entry>
	</row>
	<row>
	 <entry><literal>WIN</literal></entry>
	 <entry>Windows CP1251</entry>
	</row>
	<row>
	 <entry><literal>ALT</literal></entry>
	 <entry>Windows CP866</entry>
	</row>
	<row>
	 <entry><literal>WIN1256</literal></entry>
	 <entry>Windows CP1256 (Arabic)</entry>
	</row>
	<row>
	 <entry><literal>TCVN</literal></entry>
	 <entry><acronym>TCVN</>-5712/Windows CP1258 (Vietnamese)</entry>
	</row>
	<row>
	 <entry><literal>WIN874</literal></entry>
	 <entry>Windows CP874 (Thai)</entry>
	</row>
       </tbody>
      </tgroup>
     </table>

    <important>
     <para>
      Before <productname>PostgreSQL</> 7.2, <literal>LATIN5</>
      mistakenly meant ISO 8859-5.  From 7.2 on, <literal>LATIN5</>
      means ISO 8859-9. If you have a <literal>LATIN5</> database
      created on 7.1 or earlier and want to migrate to 7.2 or later,
      you should be careful about this change.
     </para>
    </important>

     <para>
      Not all <acronym>API</>s support all the listed character sets. For example, the
      <productname>PostgreSQL</>
      JDBC driver does not support <literal>MULE_INTERNAL</>, <literal>LATIN6</>,
      <literal>LATIN8</>, and <literal>LATIN10</>.
     </para>
    </sect2>
    
   <sect2>
    <title>Setting the Character Set</title>

    <para>
     <command>initdb</> defines the default character set
     for a <productname>PostgreSQL</productname> cluster. For example,