postgresql/doc/src/sgml/ref/create_language.sgml

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doc/src/sgml/ref/create_language.sgml
PostgreSQL documentation
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<refentry id="sql-createlanguage">
 <indexterm zone="sql-createlanguage">
  <primary>CREATE LANGUAGE</primary>
 </indexterm>

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

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

 <refsynopsisdiv>
<synopsis>
CREATE [ OR REPLACE ] [ TRUSTED ] [ PROCEDURAL ] LANGUAGE <replaceable class="parameter">name</replaceable>
    HANDLER <replaceable class="parameter">call_handler</replaceable> [ INLINE <replaceable class="parameter">inline_handler</replaceable> ] [ VALIDATOR <replaceable>valfunction</replaceable> ]
CREATE [ OR REPLACE ] [ TRUSTED ] [ PROCEDURAL ] LANGUAGE <replaceable class="parameter">name</replaceable>
</synopsis>
 </refsynopsisdiv>

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

  <para>
   <command>CREATE LANGUAGE</command> registers a new
   procedural language with a <productname>PostgreSQL</productname>
   database.  Subsequently, functions and procedures can be
   defined in this new language.
  </para>

  <para>
   <command>CREATE LANGUAGE</command> effectively associates the
   language name with handler function(s) that are responsible for executing
   functions written in the language.  Refer to <xref linkend="plhandler"/>
   for more information about language handlers.
  </para>

  <para>
   <command>CREATE OR REPLACE LANGUAGE</command> will either create a
   new language, or replace an existing definition.  If the language
   already exists, its parameters are updated according to the command,
   but the language's ownership and permissions settings do not change,
   and any existing functions written in the language are assumed to still
   be valid.
  </para>

  <para>
   One must have the
   <productname>PostgreSQL</productname> superuser privilege to
   register a new language or change an existing language's parameters.
   However, once the language is created it is valid to assign ownership of
   it to a non-superuser, who may then drop it, change its permissions,
   rename it, or assign it to a new owner.  (Do not, however, assign
   ownership of the underlying C functions to a non-superuser; that would
   create a privilege escalation path for that user.)
  </para>

  <para>
   The form of <command>CREATE LANGUAGE</command> that does not supply
   any handler function is obsolete.  For backwards compatibility with
   old dump files, it is interpreted as <command>CREATE EXTENSION</command>.
   That will work if the language has been packaged into an extension of
   the same name, which is the conventional way to set up procedural
   languages.
  </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 language does
       not grant access to data that the user would not otherwise
       have.  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">name</replaceable></term>

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

      <para>
       For backward compatibility, the name can 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's 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>INLINE</literal> <replaceable class="parameter">inline_handler</replaceable></term>

     <listitem>
      <para><replaceable class="parameter">inline_handler</replaceable> is the
       name of a previously registered function that will be called
       to execute an anonymous code block
       (<xref linkend="sql-do"/> command)
       in this language.
       If no <replaceable class="parameter">inline_handler</replaceable>
       function is specified, the language does not support anonymous code
       blocks.
       The handler function must take one argument of
       type <type>internal</type>, which will be the <command>DO</command> command's
       internal representation, and it will typically return
       <type>void</type>.  The return value of the handler is ignored.
      </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</type>.
      </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>ereport()</function>
       function.  The return value of the function is ignored.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
 </refsect1>

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

  <para>
   Use <xref linkend="sql-droplanguage"/> 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, the <application>psql</application>
   command <command>\dL</command> lists the installed languages.
  </para>

  <para>
   To create functions in a procedural language, a user must have the
   <literal>USAGE</literal> privilege for the language.  By default,
   <literal>USAGE</literal> is granted to <literal>PUBLIC</literal> (i.e., everyone)
   for trusted languages.  This can be revoked if desired.
  </para>

  <para>
   Procedural languages are local to individual databases.
   However, a language can be installed into the <literal>template1</literal>
   database, which will cause it to be available automatically in
   all subsequently-created databases.
  </para>

  <para>
   In <productname>PostgreSQL</productname> versions before 7.3, it was
   necessary to declare handler functions as returning the placeholder
   type <type>opaque</type>, rather than <type>language_handler</type>.
   To support loading
   of old dump files, <command>CREATE LANGUAGE</command> will accept a function
   declared as returning <type>opaque</type>, but it will issue a notice and
   change the function's declared return type to <type>language_handler</type>.
  </para>
 </refsect1>

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

  <para>
   A minimal sequence for creating a new procedural language is:
<programlisting>
CREATE FUNCTION plsample_call_handler() RETURNS language_handler
    AS '$libdir/plsample'
    LANGUAGE C;
CREATE LANGUAGE plsample
    HANDLER plsample_call_handler;
</programlisting>
   Typically that would be written in an extension's creation script,
   and users would do this to install the extension:
<programlisting>
CREATE EXTENSION plsample;
</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>

  <simplelist type="inline">
   <member><xref linkend="sql-alterlanguage"/></member>
   <member><xref linkend="sql-createfunction"/></member>
   <member><xref linkend="sql-droplanguage"/></member>
   <member><xref linkend="sql-grant"/></member>
   <member><xref linkend="sql-revoke"/></member>
  </simplelist>
 </refsect1>
</refentry>