This package was originally written by Jan Wieck. This
documentation was in part written
by Roberto Mello (<rmello@fslc.usu.edu>).
The design goals of PL/pgSQL were to create a loadable procedural
language that
can be used to create functions and trigger procedures,
adds control structures to the SQL language,
can perform complex computations,
inherits all user defined types, functions and operators,
can be defined to be trusted by the server,
is easy to use.
The PL/pgSQL call handler parses the function's source text and
produces an internal binary instruction tree the first time the
function is called (within any one backend process). The instruction tree
fully translates the
PL/pgSQL statement structure, but individual
SQL expressions and SQL queries
used in the function are not translated immediately.
As each expression and SQL query is first used
in the function, the PL/pgSQL interpreter creates
a prepared execution plan (using the SPI
manager's SPI_prepare and
SPI_saveplan functions). Subsequent visits
to that expression or query re-use the prepared plan. Thus, a
function with conditional code that contains many statements for
which execution plans might be required will only prepare and save
those plans that are really used during the lifetime of the
database connection. This can substantially reduce the total
amount of time required to parse, and generate query plans for the
statements in a procedural language function. A disadvantage is
that errors in a specific expression or query may not be detected
until that part of the function is reached in execution.
Once PL/pgSQL has made a query plan for a particular
query in a function, it will re-use that plan for the life of the
database connection. This is usually a win for performance, but it
can cause some problems if you dynamically
alter your database schema. For example:
CREATE FUNCTION populate() RETURNS INTEGER AS '
DECLARE
-- Declarations
BEGIN
PERFORM my_function();
END;
' LANGUAGE 'plpgsql';
If you execute the above function, it will reference the OID for
my_function() in the query plan produced for
the PERFORM statement. Later, if you
drop and re-create my_function(), then
populate() will not be able to find
my_function() anymore. You would then have to
re-create populate(), or at least start a new
database session so that it will be compiled afresh.
Because PL/pgSQL saves execution plans
in this way, queries that appear directly in a
PL/pgSQL function must refer to the
same tables and fields on every execution; that is, you cannot use
a parameter as the name of a table or field in a query. To get
around this restriction, you can construct dynamic queries using
the PL/pgSQL EXECUTE statement --- at
the price of constructing a new query plan on every execution.
Note: The PL/pgSQL EXECUTE statement is not
related to the EXECUTE statement supported by the
PostgreSQL backend. The backend
EXECUTE statement cannot be used within PL/pgSQL functions (and
is not needed).
Except for input/output conversion and calculation functions
for user defined types, anything that can be defined in C language
functions can also be done with PL/pgSQL. It is possible to
create complex conditional computation functions and later use
them to define operators or use them in functional indexes.
SQL is the language
PostgreSQL (and most other relational databases)
use as query language. It's portable and easy to learn. But
every SQL statement must be executed
individually by the database server.
That means that your client application must send each
query to the database server, wait for it to process it,
receive the results, do some computation, then send
other queries to the server. All this incurs inter-process communication
and may also incur network
overhead if your client is on a different machine than
the database server.
With PL/pgSQL you can group a block of computation and a
series of queries inside the
database server, thus having the power of a procedural
language and the ease of use of SQL, but saving lots of
time because you don't have the whole client/server
communication overhead. This can make for a
considerable performance increase.
PL/pgSQL adds the power of a procedural language to the
flexibility and ease of SQL. With
PL/pgSQL you can use all the data types, columns, operators
and functions of SQL.
Because PL/pgSQL functions run inside
PostgreSQL, these functions will run on any
platform where PostgreSQL runs. Thus you can
reuse code and reduce development costs.
Developing in PL/pgSQL is pretty straight forward, especially
if you have developed in other database procedural languages,
such as Oracle's PL/SQL. Two good ways of developing in
PL/pgSQL are:
One good way to develop in PL/pgSQL is to simply
use the text editor of your choice to create your functions, and
in another window, use psql
(PostgreSQL's interactive monitor) to load those
functions. If you are doing it this way, it is a good idea to
write the function using CREATE OR REPLACE
FUNCTION. That way you can reload the file to update the
function definition. For example:
CREATE OR REPLACE FUNCTION testfunc(INTEGER) RETURNS INTEGER AS '
....
end;
' LANGUAGE 'plpgsql';
While running psql, you can load or reload such a
function definition file with
\i filename.sql
and then immediately issue SQL commands to test the function.
Another good way to develop in PL/pgSQL is using
PostgreSQL's GUI tool: PgAccess. It does some
nice things for you, like escaping single-quotes, and making
it easy to recreate and debug functions.
