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CREATE LANGUAGE

Name

CREATE LANGUAGE  --  Defines a new language for functions

Synopsis

CREATE [ TRUSTED ] [ PROCEDURAL ] LANGUAGE 'langname'
    HANDLER call_handler
    LANCOMPILER 'comment'
  

Inputs

TRUSTED

TRUSTED specifies that the call handler for the language is safe; that is, it offers an unprivileged user no functionality to bypass access restrictions. If this keyword is omitted when registering the language, only users with the Postgres superuser privilege can use this language to create new functions.

langname

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 Postgres.

HANDLER call_handler

call_handler is the name of a previously registered function that will be called to execute the PL procedures.

comment

The LANCOMPILER argument is the string that will be inserted in the LANCOMPILER attribute of the new pg_language entry. At present, Postgres does not use this attribute in any way.

Outputs

CREATE

This message is returned if the language is successfully created.

ERROR: PL handler function funcname() doesn't exist

This error is returned if the function funcname() is not found.

Description

Using CREATE LANGUAGE, a Postgres user can register a new language with Postgres. Subsequently, functions and trigger procedures can be defined in this new language. The user must have the Postgres superuser privilege to register a new language.

Writing PL handlers

Note: In Postgres 7.1 and later, call handlers must adhere to the "version 1" function manager interface, not the old-style interface.

The call handler for a procedural language must be written in a compiled language such as C and registered with Postgres as a function taking no arguments and returning the opaque type, a placeholder for unspecified or undefined types. This prevents the call handler from being called directly as a function from queries. (However, arguments may be supplied in the actual call when a PL function in the language offered by the handler is to be executed.)

The call handler is called in the same way as any other function: it receives a pointer to a FunctionCallInfoData struct containing argument values and information about the called function, and it is expected to return a Datum result (and possibly set the isnull field of the FunctionCallInfoData struct, if it wishes to return an SQL NULL result). The difference between a call handler and an ordinary callee function is that the flinfo->fn_oid field of the FunctionCallInfoData struct will contain the OID of the PL function to be called, not of the call handler itself. The call handler must use this field to determine which function to execute. Also, the passed argument list has been set up according to the declaration of the target PL function, not of the call handler.

It's up to the call handler to fetch the pg_proc entry and to analyze the argument and return types of the called procedure. The AS clause from the CREATE FUNCTION of the procedure will be found in the prosrc attribute of the pg_proc table 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.

Often, the same function is called many times per SQL statement. A call handler can avoid repeated lookups of information about the called function by using the flinfo->fn_extra field. This will initially be NULL, but can be set by the call handler to point at information about the PL function. On subsequent calls, if flinfo->fn_extra is already non-NULL then it can be used and the information lookup step skipped. The call handler must be careful that flinfo->fn_extra is made to point at memory that will live at least until the end of the current query, since an FmgrInfo data structure could be kept that long. One way to do this is to allocate the extra data in the memory context specified by flinfo->fn_mcxt; such data will normally have the same lifespan as the FmgrInfo itself. But the handler could also choose to use a longer-lived context so that it can cache function definition information across queries.

When a PL function is invoked as a trigger, no explicit arguments are passed, but the FunctionCallInfoData's context field points at a TriggerData node, rather than being NULL as it is in a plain function call. A PL handler should provide mechanisms for PL functions to get at the trigger information.

Notes

Use CREATE FUNCTION to create a function.

Use DROP LANGUAGE to drop procedural languages.

Refer to the table pg_language for further information:

        Table "pg_language"
   Attribute   |  Type   | Modifier
---------------+---------+----------
 lanname       | name    |
 lanispl       | boolean |
 lanpltrusted  | boolean |
 lanplcallfoid | oid     |
 lancompiler   | text    |

   lanname   | lanispl | lanpltrusted | lanplcallfoid | lancompiler
-------------+---------+--------------+---------------+-------------
 internal    | f       | f            |             0 | n/a
 C           | f       | f            |             0 | /bin/cc
 sql         | f       | f            |             0 | postgres

The call handler for a procedural language must normally be written in C and registered as 'internal' or 'C' language, depending on whether it is linked into the backend or dynamically loaded. The call handler cannot use the old-style 'C' function interface.

At present, the definitions for a procedural language cannot be changed once they have been created.

Usage

This is a template for a PL handler written in C:

#include "executor/spi.h"
#include "commands/trigger.h"
#include "utils/elog.h"
#include "fmgr.h"
#include "access/heapam.h"
#include "utils/syscache.h"
#include "catalog/pg_proc.h"
#include "catalog/pg_type.h"

PG_FUNCTION_INFO_V1(plsample_call_handler);

Datum
plsample_call_handler(PG_FUNCTION_ARGS)
{
     Datum          retval;

     if (CALLED_AS_TRIGGER(fcinfo))
     {
          /*
           * Called as a trigger procedure
           */
          TriggerData    *trigdata = (TriggerData *) fcinfo->context;

          retval = ...
     } else {
          /*
           * Called as a function
           */

          retval = ...
     }

     return retval;
}
   

Only a few thousand lines of code have to be added instead of the dots to complete the PL call handler. See CREATE FUNCTION for information on how to compile it into a loadable module.

The following commands then register the sample procedural language:

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';
   

Compatibility

SQL92

CREATE LANGUAGE is a Postgres extension. There is no CREATE LANGUAGE statement in SQL92.

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