PostgreSQL: Rolling back a transaction within a plpgsql function?

Question

Coming from the MS SQL world, I tend to make heavy use of stored procedures. I'm currently writing an application uses a lot of PostgreSQL plpgsql functions. What I'd like to do is rollback all INSERTS/UPDATES contained within a particular function if I get an exception at any point within it.

I was originally under the impression that each function is wrapped in it's own transaction and that an exception would automatically rollback everything. However, that doesn't seem to be the case. I'm wondering if I ought to be using savepoints in combination with exception handling instead? But I don't really understand the difference between a transaction and a savepoint to know if this is the best approach. Any advice please?

CREATE OR REPLACE FUNCTION do_something(
         _an_input_var int
                ) RETURNS bool AS $$
        DECLARE
                _a_variable int;
        BEGIN
                INSERT INTO tableA (col1, col2, col3)
                        VALUES (0, 1, 2);

                INSERT INTO tableB (col1, col2, col3)
                        VALUES (0, 1, 'whoops! not an integer');

                -- The exception will cause the function to bomb, but the values 
                -- inserted into "tableA" are not rolled back.    

                RETURN True;
END; $$ LANGUAGE plpgsql;

Answer 1

A function does represent a transaction. You do not have to wrap a function in BEGIN/COMMIT.

Answer 2

You can't use commit or rollback command into the function, but you can use your function into a committed transaction,

BEGIN TRANSACTION; SELECT do_something(); COMMIT;

This SQL script only commits if there are no exceptions in do_something, then, it will rolling back the transaction of the function.

Answer 3

Savepoints can be used to emulate nested transactions. Because a postgresql transaction is a sequence of statements that will either be applied or discarded, savepoints can mark points within that sequence that allow rolling back to.

Since true nested transactions are not supported, this is your best bet (and a good one at that).

Answer 4

The docs say this:

A savepoint is a special mark inside a transaction that allows all commands that are executed after it was established to be rolled back, restoring the transaction state to what it was at the time of the savepoint.

They give examples too.

Edit:

You need to wrap a transaction in BEGIN and COMMIT commands.

a transaction is set up by surrounding the SQL commands of the transaction with BEGIN and COMMIT commands