Procedural Languages {#xplang}
====================

procedural language

PostgreSQL allows user-defined functions to be written in other
languages besides SQL and C. These other languages are generically
called procedural languages (PLs). For a function written in a
procedural language, the database server has no built-in knowledge about
how to interpret the function\'s source text. Instead, the task is
passed to a special handler that knows the details of the language. The
handler could either do all the work of parsing, syntax analysis,
execution, etc. itself, or it could serve as "glue" between PostgreSQL
and an existing implementation of a programming language. The handler
itself is a C language function compiled into a shared object and loaded
on demand, just like any other C function.

There are currently four procedural languages available in the standard
PostgreSQL distribution: PL/pgSQL ([???](#plpgsql)), PL/Tcl
([???](#pltcl)), PL/Perl ([???](#plperl)), and PL/Python
([???](#plpython)). There are additional procedural languages available
that are not included in the core distribution.
[???](#external-projects) has information about finding them. In
addition other languages can be defined by users; the basics of
developing a new procedural language are covered in [???](#plhandler).

Installing Procedural Languages {#xplang-install}
===============================

A procedural language must be "installed" into each database where it is
to be used. But procedural languages installed in the database
`template1` are automatically available in all subsequently created
databases, since their entries in `template1` will be copied by
`CREATE DATABASE`. So the database administrator can decide which
languages are available in which databases and can make some languages
available by default if desired.

For the languages supplied with the standard distribution, it is only
necessary to execute `CREATE EXTENSION` language\_name to install the
language into the current database. The manual procedure described below
is only recommended for installing languages that have not been packaged
as extensions.

A procedural language is installed in a database in five steps, which
must be carried out by a database superuser. In most cases the required
SQL commands should be packaged as the installation script of an
"extension", so that `CREATE EXTENSION` can be used to execute them.

The shared object for the language handler must be compiled and
installed into an appropriate library directory. This works in the same
way as building and installing modules with regular user-defined C
functions does; see [???](#dfunc). Often, the language handler will
depend on an external library that provides the actual programming
language engine; if so, that must be installed as well.

The handler must be declared with the command CREATE FUNCTION
handler\_function\_name() RETURNS language\_handler AS
\'path-to-shared-object\' LANGUAGE C; The special return type of
`language_handler` tells the database system that this function does not
return one of the defined SQL data types and is not directly usable in
SQL statements.

Optionally, the language handler can provide an "inline" handler
function that executes anonymous code blocks ([`DO`](#sql-do) commands)
written in this language. If an inline handler function is provided by
the language, declare it with a command like CREATE FUNCTION
inline\_function\_name(internal) RETURNS void AS
\'path-to-shared-object\' LANGUAGE C;

Optionally, the language handler can provide a "validator" function that
checks a function definition for correctness without actually executing
it. The validator function is called by `CREATE FUNCTION` if it exists.
If a validator function is provided by the language, declare it with a
command like CREATE FUNCTION validator\_function\_name(oid) RETURNS void
AS \'path-to-shared-object\' LANGUAGE C STRICT;

Finally, the PL must be declared with the command CREATE \[TRUSTED\]
LANGUAGE language\_name HANDLER handler\_function\_name \[INLINE
inline\_function\_name\] \[VALIDATOR validator\_function\_name\] ; The
optional key word `TRUSTED` specifies that the language does not grant
access to data that the user would not otherwise have. Trusted languages
are designed for ordinary database users (those without superuser
privilege) and allows them to safely create functions and procedures.
Since PL functions are executed inside the database server, the
`TRUSTED` flag should only be given for languages that do not allow
access to database server internals or the file system. The languages
PL/pgSQL, PL/Tcl, and PL/Perl are considered trusted; the languages
PL/TclU, PL/PerlU, and PL/PythonU are designed to provide unlimited
functionality and should *not* be marked trusted.

[example\_title](#xplang-install-example) shows how the manual
installation procedure would work with the language PL/Perl.

The following command tells the database server where to find the shared
object for the PL/Perl language\'s call handler function:

    CREATE FUNCTION plperl_call_handler() RETURNS language_handler AS
        '$libdir/plperl' LANGUAGE C;

PL/Perl has an inline handler function and a validator function, so we
declare those too:

    CREATE FUNCTION plperl_inline_handler(internal) RETURNS void AS
        '$libdir/plperl' LANGUAGE C STRICT;

    CREATE FUNCTION plperl_validator(oid) RETURNS void AS
        '$libdir/plperl' LANGUAGE C STRICT;

The command:

    CREATE TRUSTED LANGUAGE plperl
        HANDLER plperl_call_handler
        INLINE plperl_inline_handler
        VALIDATOR plperl_validator;

then defines that the previously declared functions should be invoked
for functions and procedures where the language attribute is `plperl`.

In a default PostgreSQL installation, the handler for the PL/pgSQL
language is built and installed into the "library" directory;
furthermore, the PL/pgSQL language itself is installed in all databases.
If Tcl support is configured in, the handlers for PL/Tcl and PL/TclU are
built and installed in the library directory, but the language itself is
not installed in any database by default. Likewise, the PL/Perl and
PL/PerlU handlers are built and installed if Perl support is configured,
and the PL/PythonU handler is installed if Python support is configured,
but these languages are not installed by default.
