PostgreSQL
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# CREATE LANGUAGE

## Name

CREATE LANGUAGE — Defines a new language for functions

    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 (like the 'C' language).

`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

The call handler for a procedural language must be written in a compiler 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 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.

- When called from the trigger manager, the only argument is the object ID from the procedure's `pg_proc` entry. All other information from the trigger manager is found in the global CurrentTriggerData pointer.

- When called from the function manager, the arguments are the object ID of the procedure's `pg_proc` entry, the number of arguments given to the PL function, the arguments in a FmgrValues structure and a pointer to a boolean where the function tells the caller if the return value is the SQL NULL value.

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.

### 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

Since the call handler for a procedural language must be registered with Postgres in the 'C' language, it inherits all the capabilities and restrictions of 'C' functions.

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"        /* 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;
    }

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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