19.2. Structure of PL/pgSQL

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;

Any statement in the statement section of a block can be a sub-block. Sub-blocks 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 sub-block
    --
    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 transactions, since PostgreSQL does not have nested transactions.