Procedural Languages
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 (PL>s). 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 (),
PL/Tcl (),
PL/Perl (), and
PL/Python ().
Other languages can be defined by users.
The basics of developing a new procedural language are covered in .
There are additional procedural languages available that are not
included in the core distribution.
has information about finding them.
Installing Procedural Languages
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 he chooses.
For the languages supplied with the standard distribution, it is
only necessary to execute CREATE LANGUAGE>
language_name> to install the language into the
current database. Alternatively, the program may be used to do this from the shell
command line. For example, to install the language
PL/pgSQL into the database
template1>, use
createlang plpgsql template1
The manual procedure described below is only recommended for
installing custom languages that CREATE LANGUAGE
does not know about.
Manual Procedural Language Installation
A procedural language is installed in a database in four steps,
which must be carried out by a database superuser. (For languages
known to CREATE LANGUAGE>, the second and third steps
can be omitted, because they will be carried out automatically
if needed.)
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 . 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 may 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 handler, declare it with a command like
CREATE FUNCTION validator_function_name(oid)
RETURNS void
AS 'path-to-shared-object'
LANGUAGE C;
The PL must be declared with the command
CREATE TRUSTED PROCEDURAL LANGUAGE language-name
HANDLER handler_function_name
VALIDATOR validator_function_name ;
The optional key word TRUSTED specifies that
ordinary database users that have no superuser privileges should
be allowed to use this language to create functions and trigger
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.
shows how the manual
installation procedure would work with the language
PL/pgSQL.
Manual Installation of PL/pgSQL
The following command tells the database server where to find the
shared object for the PL/pgSQL language's call handler function.
CREATE FUNCTION plpgsql_call_handler() RETURNS language_handler AS
'$libdir/plpgsql' LANGUAGE C;
PL/pgSQL has a validator function,
so we declare that too:
CREATE FUNCTION plpgsql_validator(oid) RETURNS void AS
'$libdir/plpgsql' LANGUAGE C;
The command
CREATE TRUSTED PROCEDURAL LANGUAGE plpgsql
HANDLER plpgsql_call_handler
VALIDATOR plpgsql_validator;
then defines that the previously declared functions
should be invoked for functions and trigger procedures where the
language attribute is plpgsql.
In a default PostgreSQL installation,
the handler for the PL/pgSQL language
is built and installed into the library
directory. If Tcl> support is configured in, the handlers
for PL/Tcl> and PL/TclU> are also built and
installed in the same location. 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.