Functions written in PL/pgSQL are defined to the server by executing CREATE FUNCTION commands. Such a command would normally look like, say,
CREATE FUNCTION somefunc(integer, text) RETURNS integer
AS 'function body text'
LANGUAGE plpgsql;
The function body is simply a string literal so far as CREATE
FUNCTION is concerned. It is often helpful to use dollar quoting
(see Section 4.1.2.4) to write the function
body, rather than the normal single quote syntax. Without dollar quoting,
any single quotes or backslashes in the function body must be escaped by
doubling them. Almost all the examples in this chapter use dollar-quoted
literals for their function bodies.
PL/pgSQL is a block-structured language. The complete text of a function body must be a block. A block is defined as:
[ <<label>> ] [ DECLAREdeclarations] BEGINstatementsEND [label];
Each declaration and each statement within a block is terminated
by a semicolon. A block that appears within another block must
have a semicolon after END, as shown above;
however the final END that
concludes a function body does not require a semicolon.
A common mistake is to write a semicolon immediately after
BEGIN. This is incorrect and will result in a syntax error.
A label is only needed if you want to
identify the block for use
in an EXIT statement, or to qualify the names of the
variables declared in the block. If a label is given after
END, it must match the label at the block's beginning.
All key words are case-insensitive. Identifiers are implicitly converted to lower case unless double-quoted, just as they are in ordinary SQL commands.
Comments work the same way in PL/pgSQL code as in
ordinary SQL. A double dash (--) starts a comment
that extends to the end of the line. A /* starts a
block comment that extends to the matching occurrence of
*/. Block comments nest.
Any statement in the statement section of a block can be a subblock. Subblocks can be used for logical grouping or to localize variables to a small group of statements. Variables declared in a subblock mask any similarly-named variables of outer blocks for the duration of the subblock; but you can access the outer variables anyway if you qualify their names with their block's label. For example:
CREATE FUNCTION somefunc() RETURNS integer AS $$
<< outerblock >>
DECLARE
quantity integer := 30;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 30
quantity := 50;
--
-- Create a subblock
--
DECLARE
quantity integer := 80;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 80
RAISE NOTICE 'Outer quantity here is %', outerblock.quantity; -- Prints 50
END;
RAISE NOTICE 'Quantity here is %', quantity; -- Prints 50
RETURN quantity;
END;
$$ LANGUAGE plpgsql;
There is actually a hidden “outer block” surrounding the body
of any PL/pgSQL function. This block provides the
declarations of the function's parameters (if any), as well as some
special variables such as FOUND (see
Section 42.5.5). The outer block is
labeled with the function's name, meaning that parameters and special
variables can be qualified with the function's name.
It is important not to confuse the use of
BEGIN/END for grouping statements in
PL/pgSQL with the similarly-named SQL commands
for transaction
control. PL/pgSQL's BEGIN/END
are only for grouping; they do not start or end a transaction.
See Section 42.8 for information on managing
transactions in PL/pgSQL.
Also, a block containing an EXCEPTION clause effectively
forms a subtransaction that can be rolled back without affecting the
outer transaction. For more about that see Section 42.6.8.