<REFENTRY ID="SQL-CREATELANGUAGE-1">
 <REFMETA>
  <REFENTRYTITLE>
   CREATE LANGUAGE
  </REFENTRYTITLE>
  <REFMISCINFO>SQL - Language Statements</REFMISCINFO>
 </REFMETA>
 <REFNAMEDIV>
  <REFNAME>
   CREATE LANGUAGE
  </REFNAME>
  <REFPURPOSE>
   Defines a new language for functions
  </REFPURPOSE>
  
 <REFSYNOPSISDIV>
  <REFSYNOPSISDIVINFO>
   <DATE>1998-04-15</DATE>
  </REFSYNOPSISDIVINFO>
  <SYNOPSIS>
   CREATE [TRUSTED] PROCEDURAL LANGUAGE '<replaceable class="parameter">langname</replaceable>'
        HANDLER <replaceable class="parameter">call_handler</replaceable>
        LANCOMPILER '<replaceable class="parameter">comment</replaceable>'
  </SYNOPSIS>
  
  <REFSECT2 ID="R2-SQL-CREATELANGUAGE-1">
   <REFSECT2INFO>
    <DATE>1998-04-15</DATE>
   </REFSECT2INFO>
   <TITLE>
    Inputs
   </TITLE>
   <PARA>
   </PARA>
   <VARIABLELIST>
    <VARLISTENTRY>
     <TERM>
     </TERM>
     <LISTITEM>
      <PARA>
       <VARIABLELIST>
	<VARLISTENTRY>
	 <TERM>
	  <function>TRUSTED</function>
	 </TERM>
	 <LISTITEM>
	  <PARA>
	   <function> TRUSTED</function> specifies that the call handler for
	   the language is safe; that is, it offers an unprivileged user
	   no functionality to get around access restrictions. If
	   this keyword is omitted when registering the language,
	   only users with the PostgreSQL superuser privilege can use
	   this language to create new functions
	   (like the 'C' language).  
	  </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. A procedural 
	   language cannot override one of the built-in languages of
	   PostgreSQL.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY> 
	<VARLISTENTRY>
	 <TERM>
	  <replaceable class="parameter">call_handler</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
	   The argument for <function>HANDLER</function> is the name
	   of a previously
	   registered function that will be called to execute the PL
	   procedures.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
	<VARLISTENTRY>
	 <TERM>
	  <replaceable class="parameter">comment</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
	   The <function>LANCOMPILER</function> argument is the
	   string that will be
	   inserted in the <literal>LANCOMPILER</literal> attribute
	   of the new
	   <filename>pg_language</filename> entry. At present,
	   PostgreSQL doesn't use
	   this attribute in any way.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
       </variablelist>
     </LISTITEM>
    </VARLISTENTRY>
   </VARIABLELIST>
  </REFSECT2>
  
  <REFSECT2 ID="R2-SQL-CREATELANGUAGE-2">
   <REFSECT2INFO>
    <DATE>1998-04-15</DATE>
   </REFSECT2INFO>
   <TITLE>
    Outputs
   </TITLE>
   <PARA>
   </PARA>
   <VARIABLELIST>
    <VARLISTENTRY>
     <TERM>
     </TERM>
     <LISTITEM>
      <PARA>
       <VARIABLELIST>
	<VARLISTENTRY>
	 <TERM>
	  <ReturnValue>CREATE</ReturnValue>
	 </TERM>
	 <LISTITEM>
	  <PARA>
	   This message is returned if the language is successfully
	   created.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
	<VARLISTENTRY>
	 <TERM>
	  <ReturnValue>ERROR:  PL handler function <replaceable class="parameter">funcname</replaceable>() doesn't exist</ReturnValue>
	 </TERM>
	 <LISTITEM>
	  <PARA>
           This error is returned if the function
	   <replaceable class="parameter">funcname</replaceable>()
	   is not found.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
       </variablelist>
     </LISTITEM>
    </VARLISTENTRY>
   </VARIABLELIST>
  </REFSECT2>
 </REFSYNOPSISDIV>
 
 <REFSECT1 ID="R1-SQL-CREATELANGUAGE-1">
  <REFSECT1INFO>
   <DATE>1998-04-15</DATE>
  </REFSECT1INFO>
  <TITLE>
   Description
  </TITLE>
  <PARA>
   Using <command>CREATE LANGUAGE</command>, a PostgreSQL user can register
   a new language with PostgreSQL. Subsequently, functions and
   trigger procedures can be defined in this new language.
   The user must have the PostgreSQL superuser privilege to
   register a new language.
  </PARA>
  
  <REFSECT2 ID="R2-SQL-CREATELANGUAGE-3">
   <REFSECT2INFO>
    <DATE>1998-04-15</DATE>
   </REFSECT2INFO>
   <TITLE>
    Writing PL handlers
   </TITLE>
   <PARA>
    The call handler for a procedural language must be written
    in a compiler language such as 'C' and registered with
    PostgreSQL as a function taking no arguments and returning
    opaque type.
    <comment>What does `opaque type' mean?</comment>
    This prevents the call handler from being
    called directly as a function from queries.
   </para>
   <para>
    However, arguments must be supplied on the actual call when a
    PL function or trigger
    procedure in the language offered by the handler is to be
    executed.
    <itemizedlist>
     <listitem>
      <para>
       When called from the trigger manager, the only argument is
       the object ID from the procedure's <filename>pg_proc</filename>
       entry. All other
       information from the trigger manager is found in the
       global <structname>CurrentTriggerData</structname> pointer.
      </para>
     </listitem>
     <listitem>
      <para>
       When called from the function manager, the arguments are
       the object ID of the procedure's <filename>pg_proc</filename>
       entry, the number
       of arguments given to the PL function, the arguments in a
       <structname>FmgrValues</structname> structure and a pointer
       to a boolean where the
       function tells the caller if the return value is the SQL
       NULL value.
      </para>
     </listitem>
    </itemizedlist>
   </PARA>
   <para>
    It's up to the call handler to fetch the
    <filename>pg_proc</filename> entry and
    to analyze the argument and return types of the called
    procedure. The <function>AS</function> clause from the
    <command>CREATE FUNCTION</command> of
    the procedure will be found in the <literal>prosrc</literal>
    attribute of the
    <filename>pg_proc</filename> entry. This may be the
    source text in the procedural
    language itself (like for PL/Tcl), a pathname to a
    file or anything else that tells the call handler what to
    do in detail.
   </para>
   
  <REFSECT2 ID="R2-SQL-CREATELANGUAGE-4">
   <REFSECT2INFO>
    <DATE>1998-04-15</DATE>
   </REFSECT2INFO>
   <TITLE>
    Notes
   </TITLE>
   <PARA>
    Use <citerefentry>
     <refentrytitle>CREATE FUNCTION</refentrytitle>
    </citerefentry>
    to create a function.
   </para>
   <para>
    Use <citerefentry>
     <refentrytitle>DROP LANGUAGE</refentrytitle>
    </citerefentry>  to drop procedural languages.
   </para>
   <para>
    Refer to the table <filename>pg_language</filename>
    for further information:
    <programlisting>
     <computeroutput>
     Table    = pg_language
     +--------------------------+--------------------------+-------+
     |          Field           |          Type            | Length|
     +--------------------------+--------------------------+-------+
     | lanname                  | name                     |    32 |
     | lancompiler              | text                     |   var |
     +--------------------------+--------------------------+-------+

     lanname |lancompiler   
     --------+--------------
     internal|n/a           
     lisp    |/usr/ucb/liszt
     C       |/bin/cc       
     sql     |postgres
     </computeroutput>
    </programlisting>
   </para>
  </refsect2>
 </refsect1>
 
 <REFSECT1 ID="R1-SQL-CREATELANGUAGE-4">
  <TITLE>
   Restrictions
  </TITLE>
  <PARA>
   Since the call handler for a procedural language must be
   registered with PostgreSQL in the 'C' language, it inherits
   all the restrictions of 'C' functions.
   <comment>
    What are these restrictions?
   </comment>
  </para>
 </refsect1>
 <REFSECT1 ID="R1-SQL-CREATELANGUAGE-5">
  <TITLE>
   Bugs
  </TITLE>
  <PARA> 
   At present, the definitions for a procedural language cannot be
   changed once they have been created.
  </para>
 </refsect1>
 
 <REFSECT1 ID="R1-SQL-CREATELANGUAGE-6">
  <TITLE>
   Usage
  </TITLE>
  <PARA>
   This is a template for a PL handler written in 'C':
  </PARA>
  <ProgramListing>
   #include "executor/spi.h"
   #include "commands/trigger.h"
   #include "utils/elog.h"
   #include "fmgr.h"        /* for FmgrValues struct */
   #include "access/heapam.h"
   #include "utils/syscache.h"
   #include "catalog/pg_proc.h"
   #include "catalog/pg_type.h"
        
   Datum
   plsample_call_handler(
        Oid       prooid,
        int       pronargs,
        FmgrValues     *proargs,
        bool      *isNull)
   {
        Datum          retval;
        TriggerData    *trigdata;

        if (CurrentTriggerData == NULL) {
             /*
              * Called as a function
              */

             retval = ...
        } else {
             /*
              * Called as a trigger procedure
              */
             trigdata = CurrentTriggerData;
             CurrentTriggerData = NULL;

             retval = ...
        }

        *isNull = false;
        return retval;
   }
  </ProgramListing>
  <para>
   Only a few thousand lines of code have to be added instead
   of the dots to complete the PL call handler. See <citerefentry>
    <refentrytitle>CREATE FUNCTION</refentrytitle>
   </citerefentry> for information on how to compile
   it into a loadable module
.</para>
  <para>
   The following commands then register the sample procedural
   language.</para>
  <programlisting>
   CREATE FUNCTION plsample_call_handler () RETURNS opaque
        AS '/usr/local/pgsql/lib/plsample.so'
        LANGUAGE 'C';
        
   CREATE PROCEDURAL LANGUAGE 'plsample'
        HANDLER plsample_call_handler
        LANCOMPILER 'PL/Sample';
  </programlisting>
 </REFSECT1>
 
 <REFSECT1 ID="R1-SQL-CREATELANGUAGE-7">
  <TITLE>
   Compatibility
  </TITLE>
  <PARA>
   CREATE LANGUAGE is a PostgreSQL extension.
  </PARA>
  
  <REFSECT2 ID="R2-SQL-CREATELANGUAGE-5">
   <REFSECT2INFO>
    <DATE>1998-04-15</DATE>
   </REFSECT2INFO>
   <TITLE>
    SQL92
   </TITLE>
   <PARA>
    There is no CREATE LANGUAGE statement in SQL92.
   </PARA>
  </refsect2>
 </refsect1>
</REFENTRY>


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