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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 (Chapter 37, PL/pgSQL - SQL Procedural Language), PL/Tcl (Chapter 38, PL/Tcl - Tcl Procedural Language), PL/Perl (Chapter 39, PL/Perl - Perl Procedural Language), and PL/Python (Chapter 40, PL/Python - Python Procedural Language). There are additional procedural languages available that are not included in the core distribution. Appendix H, 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 Chapter 47, Writing A Procedural Language Handler.
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 createlang 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 Section 33.9.6, “Compiling and Linking Dynamically-Loaded Functions”. 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 FUNCTIONhandler_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 FUNCTIONvalidator_function_name
(oid) RETURNS void AS 'path-to-shared-object
' LANGUAGE C;
The PL must be declared with the command
CREATE [TRUSTED] [PROCEDURAL] LANGUAGElanguage-name
HANDLERhandler_function_name
[VALIDATORvalidator_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.
Example 36.1, “Manual Installation of PL/pgSQL” shows how the manual installation procedure would work with the language PL/pgSQL.
Example 36.1. 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.