PL/pgSQL is a block-structured language. The complete text of a function definition must be a block. A block is defined as:
[ <<
label
>> ]
[ DECLARE
declarations
]
BEGIN
statements
END [
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.
All key words and identifiers can be written in mixed upper and lower case. Identifiers are implicitly converted to lowercase unless double-quoted.
There are two types of comments in PL/pgSQL. A double dash (--) starts a comment that extends to the end of the line. A /* starts a block comment that extends to the next occurrence of */. Block comments cannot be nested, but double dash comments can be enclosed into a block comment and a double dash can hide the block comment delimiters /* and */.
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.
The variables declared in the declarations section preceding a block are initialized to their default values every time the block is entered, not only once per function call. For example:
CREATE FUNCTION somefunc() RETURNS integer AS $$
DECLARE
quantity integer := 30;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Quantity here is 30
quantity := 50;
--
-- Create a subblock
--
DECLARE
quantity integer := 80;
BEGIN
RAISE NOTICE 'Quantity here is %', quantity; -- Quantity here is 80
END;
RAISE NOTICE 'Quantity here is %', quantity; -- Quantity here is 50
RETURN quantity;
END;
$$ LANGUAGE plpgsql;
It is important not to confuse the use of BEGIN/END for grouping statements in PL/pgSQL with the database commands for transaction control. PL/pgSQL's BEGIN/END are only for grouping; they do not start or end a transaction. Functions and trigger procedures are always executed within a transaction established by an outer query — they cannot start or commit that transaction, since there would be no context for them to execute in. However, 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 36.7.5.
