PostgreSQL 8.2.23 Documentation
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# 37.3. 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 [ 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 37.7.5.

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