postgres/doc/src/sgml/ref/create_conversion.sgml

<!--
doc/src/sgml/ref/create_conversion.sgml
PostgreSQL documentation
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<refentry id="sql-createconversion">
 <indexterm zone="sql-createconversion">
  <primary>CREATE CONVERSION</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE CONVERSION</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE CONVERSION</refname>
  <refpurpose>define a new encoding conversion</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE [ DEFAULT ] CONVERSION <replaceable>name</replaceable>
    FOR <replaceable>source_encoding</replaceable> TO <replaceable>dest_encoding</replaceable> FROM <replaceable>function_name</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1 id="sql-createconversion-description">
  <title>Description</title>

  <para>
   <command>CREATE CONVERSION</command> defines a new conversion between
   two character set encodings.
  </para>

  <para>
   Conversions that are marked <literal>DEFAULT</literal> can be used for
   automatic encoding conversion between client and server.  To support that
   usage, two conversions, from encoding A to B <emphasis>and</emphasis>
   from encoding B to A, must be defined.
  </para>

  <para>
   To be able to create a conversion, you must have <literal>EXECUTE</literal> privilege
   on the function and <literal>CREATE</literal> privilege on the destination schema.
  </para>
 </refsect1>


 <refsect1>
  <title>Parameters</title>

   <variablelist>
    <varlistentry>
     <term><literal>DEFAULT</literal></term>

     <listitem>
      <para>
       The <literal>DEFAULT</literal> clause indicates that this conversion
       is the default for this particular source to destination
       encoding. There should be only one default encoding in a schema
       for the encoding pair.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>name</replaceable></term>

     <listitem>
      <para>
       The name of the conversion. The conversion name can be
       schema-qualified. If it is not, the conversion is defined in the
       current schema. The conversion name must be unique within a
       schema.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>source_encoding</replaceable></term>

     <listitem>
      <para>
       The source encoding name.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>dest_encoding</replaceable></term>

     <listitem>
      <para>
       The destination encoding name.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>function_name</replaceable></term>

     <listitem>
      <para>
       The function used to perform the conversion.  The function name can
       be schema-qualified.  If it is not, the function will be looked
       up in the path.
      </para>

      <para>
       The function must have the following signature:

<programlisting>
conv_proc(
    integer,  -- source encoding ID
    integer,  -- destination encoding ID
    cstring,  -- source string (null terminated C string)
    internal, -- destination (fill with a null terminated C string)
    integer,  -- source string length
    boolean   -- if true, don't throw an error if conversion fails
) RETURNS integer;
</programlisting>
       The return value is the number of source bytes that were successfully
       converted. If the last argument is false, the function must throw an
       error on invalid input, and the return value is always equal to the
       source string length.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
 </refsect1>

 <refsect1 id="sql-createconversion-notes">
  <title>Notes</title>

  <para>
   Neither the source nor the destination encoding can
   be <literal>SQL_ASCII</literal>, as the server's behavior for cases
   involving the <literal>SQL_ASCII</literal> <quote>encoding</quote> is
   hard-wired.
  </para>

  <para>
   Use <command>DROP CONVERSION</command> to remove user-defined conversions.
  </para>

  <para>
   The privileges required to create a conversion might be changed in a future
   release.
  </para>
 </refsect1>

 <refsect1 id="sql-createconversion-examples">
  <title>Examples</title>

  <para>
   To create a conversion from encoding <literal>UTF8</literal> to
   <literal>LATIN1</literal> using <function>myfunc</function>:
<programlisting>
CREATE CONVERSION myconv FOR 'UTF8' TO 'LATIN1' FROM myfunc;
</programlisting></para>
 </refsect1>


 <refsect1 id="sql-createconversion-compat">
  <title>Compatibility</title>

  <para>
    <command>CREATE CONVERSION</command>
    is a <productname>PostgreSQL</productname> extension.
    There is no <command>CREATE CONVERSION</command>
    statement in the SQL standard, but a <command>CREATE TRANSLATION</command>
    statement that is very similar in purpose and syntax.
  </para>
 </refsect1>


 <refsect1 id="sql-createconversion-seealso">
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-alterconversion"/></member>
   <member><xref linkend="sql-createfunction"/></member>
   <member><xref linkend="sql-dropconversion"/></member>
  </simplelist>
 </refsect1>

</refentry>