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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_language.sgml,v 1.32 2003/04/22 10:08:08 petere Exp $
PostgreSQL documentation
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<refentry id="SQL-CREATELANGUAGE">
 <refmeta>
  <refentrytitle id="sql-createlanguage-title">CREATE LANGUAGE</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE LANGUAGE</refname>
  <refpurpose>define a new procedural language</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE [ TRUSTED ] [ PROCEDURAL ] LANGUAGE <replaceable class="parameter">langname</replaceable>
    HANDLER <replaceable class="parameter">call_handler</replaceable> [ VALIDATOR <replaceable>valfunction</replaceable> ]
</synopsis>
 </refsynopsisdiv>

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

  <para>
   Using <command>CREATE LANGUAGE</command>, a
   <productname>PostgreSQL</productname> user can register a new
   procedural language with a <productname>PostgreSQL</productname>
   database.  Subsequently, functions and trigger procedures can be
   defined in this new language.  The user must have the
   <productname>PostgreSQL</productname> superuser privilege to
   register a new language.
  </para>

  <para>
   <command>CREATE LANGUAGE</command> effectively associates the
   language name with a call handler that is responsible for executing
   functions written in the language.  Refer to <xref linkend="xfunc">
   for more information about language call handlers.
  </para>

  <para>
   Note that procedural languages are local to individual databases.
   To make a language available in all databases by default, it should
   be installed into the <literal>template1</literal> database.
  </para>
 </refsect1>

 <refsect1 id="sql-createlanguage-parameters">
  <title>Parameters</title>

   <variablelist>
    <varlistentry>
     <term><literal>TRUSTED</literal></term>

     <listitem>
      <para>
       <literal>TRUSTED</literal> specifies that the call handler for
       the language is safe, that is, it does not offer an
       unprivileged user any functionality to bypass access
       restrictions. If this key word is omitted when registering the
       language, only users with the
       <productname>PostgreSQL</productname> superuser privilege can
       use this language to create new functions.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>PROCEDURAL</literal></term>

     <listitem>
      <para>
       This is a noise word.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable class="parameter">langname</replaceable></term>

     <listitem>
      <para>
       The name of the new procedural language.  The language name is
       case insensitive. The name must be unique among the languages
       in the database.
      </para>

      <para>
       For backward compatibility, the name may be enclosed by single
       quotes.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>HANDLER</literal> <replaceable class="parameter">call_handler</replaceable></term>

     <listitem>
      <para>
       <replaceable class="parameter">call_handler</replaceable> is
       the name of a previously registered function that will be
       called to execute the procedural language functions.  The call
       handler for a procedural language must be written in a compiled
       language such as C with version 1 call convention and
       registered with <productname>PostgreSQL</productname> as a
       function taking no arguments and returning the
       <type>language_handler</type> type, a placeholder type that is
       simply used to identify the function as a call handler.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>VALIDATOR</literal> <replaceable class="parameter">valfunction</replaceable></term>

     <listitem>
      <para>
       <replaceable class="parameter">valfunction</replaceable> is the
       name of a previously registered function that will be called
       when a new function in the language is created, to validate the
       new function.
       If no
       validator function is specified, then a new function will not
       be checked when it is created.
       The validator function must take one argument of
       type <type>oid</type>, which will be the OID of the
       to-be-created function, and will typically return <type>void</>.
      </para>

      <para>
       A validator function would typically inspect the function body
       for syntactical correctness, but it can also look at other
       properties of the function, for example if the language cannot
       handle certain argument types.  To signal an error, the
       validator function should use the <function>elog()</function>
       function.  The return value of the function is ignored.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
 </refsect1>

 <refsect1 id="sql-createlanguage-diagnostics">
  <title>Diagnostics</title>

  <variablelist>
   <varlistentry>
    <term><computeroutput>CREATE LANGUAGE</computeroutput></term>
    <listitem>
     <para>
      Message returned if the language was successfully created.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

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

  <para>
   This command normally should not be executed directly by users.
   For the procedural languages supplied in the
   <productname>PostgreSQL</productname> distribution, the <xref
   linkend="app-createlang"> program should be used, which will also
   install the correct call handler.  (<command>createlang</command>
   will call <command>CREATE LANGUAGE</command> internally.)
  </para>

  <para>
   In <productname>PostgreSQL</productname> versions before 7.3, it was
   necessary to declare handler functions as returning the placeholder
   type <type>opaque</>, rather than <type>language_handler</>.
   To support loading 
   of old dump files, <command>CREATE LANGUAGE</> will accept a function
   declared as returning <type>opaque</>, but it will issue a notice and
   change the function's declared return type to <type>language_handler</>.
  </para>

  <para>
   Use the <xref linkend="sql-createfunction" endterm="sql-createfunction-title"> command to create a new
   function.
  </para>

  <para>
   Use <xref linkend="sql-droplanguage" endterm="sql-droplanguage-title">, or better yet the <xref
   linkend="app-droplang"> program, to drop procedural languages.
  </para>

  <para>
   The system catalog <classname>pg_language</classname> (see <xref
   linkend="catalog-pg-language">) records information about the
   currently installed languages.  Also <command>createlang</command>
   has an option to list the installed languages.
  </para>

  <para>
   The definition of a procedural language cannot be changed once it
   has been created, with the exception of the privileges.
  </para>

  <para>
   To be able to use a procedural language, a user must be granted the
   <literal>USAGE</literal> privilege.  The
   <command>createlang</command> program automatically grants
   permissions to everyone if the language is known to be trusted.
  </para>
 </refsect1>

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

  <para>
   The following two commands executed in sequence will register a new
   procedural language and the associated call handler.
<programlisting>
CREATE FUNCTION plsample_call_handler() RETURNS language_handler
    AS '$libdir/plsample'
    LANGUAGE C;
CREATE LANGUAGE plsample
    HANDLER plsample_call_handler;
</programlisting>
  </para>
 </refsect1>

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

  <para>
   <command>CREATE LANGUAGE</command> is a
   <productname>PostgreSQL</productname> extension.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <para>
   <simplelist type="inline">
    <member><xref linkend="app-createlang"></member>
    <member><xref linkend="sql-createfunction" endterm="sql-createfunction-title"></member>
    <member><xref linkend="app-droplang"></member>
    <member><xref linkend="sql-droplanguage" endterm="sql-droplanguage-title"></member>
    <member><xref linkend="sql-grant" endterm="sql-grant-title"></member>
    <member><xref linkend="sql-revoke" endterm="sql-revoke-title"></member>
   </simplelist>
  </para>
 </refsect1>
</refentry>

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