CREATE [ PROCEDURAL ] LANGUAGE name CREATE [ TRUSTED ] [ PROCEDURAL ] LANGUAGE name HANDLER call_handler [ VALIDATOR valfunction ]
Using CREATE LANGUAGE, a PostgreSQL user can register a new procedural language with a PostgreSQL database. Subsequently, functions and trigger procedures can be defined in this new language.
CREATE LANGUAGE effectively associates the language name with a call handler that is responsible for executing functions written in the language. Refer to Chapter 37 for more information about language call handlers.
There are two forms of the CREATE LANGUAGE command. In the first form, the user supplies just the name of the desired language, and the PostgreSQL server consults the pg_pltemplate system catalog to determine the correct parameters. In the second form, the user supplies the language parameters along with the language name. The second form can be used to create a language that is not defined in pg_pltemplate, but this approach is considered obsolescent.
When the server finds an entry in the pg_pltemplate catalog for the given language name, it will use the catalog data even if the command includes language parameters. This behavior simplifies loading of old dump files, which are likely to contain out-of-date information about language support functions.
Ordinarily, the user must have the PostgreSQL superuser privilege to register a new language. However, the owner of a database can register a new language within that database if the language is listed in the pg_pltemplate catalog and is marked as allowed to be created by database owners (tmpldbacreate is true). The default is that trusted languages can be created by database owners, but this can be adjusted by superusers by modifying the contents of pg_pltemplate. The creator of a language becomes its owner and can later drop it, rename it, or assign it to a new owner.
TRUSTED 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 PostgreSQL superuser privilege can use this language to create new functions.
This is a noise word.
The name of the new procedural language. The language name is case insensitive. The name must be unique among the languages in the database.
For backward compatibility, the name can be enclosed by single quotes.
call_handler 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 PostgreSQL as a function taking no arguments and returning the language_handler type, a placeholder type that is simply used to identify the function as a call handler.
valfunction 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 oid, which will be the OID of the to-be-created function, and will typically return void.
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
ereport() function. The
return value of the function is ignored.
The TRUSTED option and the support function name(s) are ignored if the server has an entry for the specified language name in pg_pltemplate.
The createlang program is a simple wrapper around the CREATE LANGUAGE command. It eases installation of procedural languages from the shell command line.
The system catalog
(see Section 44.22)
records information about the currently installed languages.
Also, createlang has an option to list
the installed languages.
To create functions in a procedural language, a user must have the USAGE privilege for the language. By default, USAGE is granted to PUBLIC (i.e., everyone) for trusted languages. This can be revoked if desired.
Procedural languages are local to individual databases. However, a language can be installed into the template1 database, which will cause it to be available automatically in all subsequently-created databases.
The call handler function and the validator function (if any) must already exist if the server does not have an entry for the language in pg_pltemplate. But when there is an entry, the functions need not already exist; they will be automatically defined if not present in the database. (This might result in CREATE LANGUAGE failing, if the shared library that implements the language is not available in the installation.)
In PostgreSQL versions before 7.3, it was necessary to declare handler functions as returning the placeholder type opaque, rather than language_handler. To support loading of old dump files, CREATE LANGUAGE will accept a function declared as returning opaque, but it will issue a notice and change the function's declared return type to language_handler.
The preferred way of creating any of the standard procedural languages is just:
CREATE LANGUAGE plpgsql;
For a language not known in the pg_pltemplate catalog, a sequence such as this is needed:
CREATE FUNCTION plsample_call_handler() RETURNS language_handler AS '$libdir/plsample' LANGUAGE C; CREATE LANGUAGE plsample HANDLER plsample_call_handler;
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