In procedures invoked by the
CALL command as well as in anonymous code blocks (
DO command), it is possible to end transactions using the commands
ROLLBACK. A new transaction is started automatically after a transaction is ended using these commands, so there is no separate
START TRANSACTION command. (Note that
END have different meanings in PL/pgSQL.)
Here is a simple example:
CREATE PROCEDURE transaction_test1() LANGUAGE plpgsql AS $$ BEGIN FOR i IN 0..9 LOOP INSERT INTO test1 (a) VALUES (i); IF i % 2 = 0 THEN COMMIT; ELSE ROLLBACK; END IF; END LOOP; END $$; CALL transaction_test1();
A new transaction starts out with default transaction characteristics such as transaction isolation level. In cases where transactions are committed in a loop, it might be desirable to start new transactions automatically with the same characteristics as the previous one. The commands
COMMIT AND CHAIN and
ROLLBACK AND CHAIN accomplish this.
Transaction control is only possible in
DO invocations from the top level or nested
DO invocations without any other intervening command. For example, if the call stack is
CALL proc1() →
CALL proc2() →
CALL proc3(), then the second and third procedures can perform transaction control actions. But if the call stack is
CALL proc1() →
SELECT func2() →
CALL proc3(), then the last procedure cannot do transaction control, because of the
SELECT in between.
Special considerations apply to cursor loops. Consider this example:
CREATE PROCEDURE transaction_test2() LANGUAGE plpgsql AS $$ DECLARE r RECORD; BEGIN FOR r IN SELECT * FROM test2 ORDER BY x LOOP INSERT INTO test1 (a) VALUES (r.x); COMMIT; END LOOP; END; $$; CALL transaction_test2();
Normally, cursors are automatically closed at transaction commit. However, a cursor created as part of a loop like this is automatically converted to a holdable cursor by the first
ROLLBACK. That means that the cursor is fully evaluated at the first
ROLLBACK rather than row by row. The cursor is still removed automatically after the loop, so this is mostly invisible to the user.
Transaction commands are not allowed in cursor loops driven by commands that are not read-only (for example
UPDATE ... RETURNING).
A transaction cannot be ended inside a block with exception handlers.