CREATE LANGUAGE — define a new procedural language
CREATE [ PROCEDURAL ] LANGUAGEname
CREATE [ TRUSTED ] [ PROCEDURAL ] LANGUAGEname
HANDLERcall_handler
[ VALIDATORvalfunction
]
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. The user must have the
PostgreSQL superuser privilege to
register a 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 36, Procedural Languages
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old dump files, which are likely to contain out-of-date information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.
TRUSTED
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.
PROCEDURAL
This is a noise word.
name
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 may be enclosed by single quotes.
HANDLER
call_handler
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.
VALIDATOR
valfunction
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.
Use DROP LANGUAGE, or better yet the droplang program, to drop procedural languages.
The system catalog pg_language
(see Section 43.20, “pg_language
”) 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 may 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 can 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;