SELECT … FOR UPDATE

Informix®

Legacy BDL programs typically use a cursor with SELECT FOR UPDATE to implement pessimistic locking and avoid several users editing the same rows:

DECLARE cc CURSOR FOR
SELECT ... FROM tab WHERE ... FOR UPDATE
OPEN cc
FETCH cc <-- lock is acquired
...
CLOSE cc <-- lock is released

The row must be fetched in order to set the lock.

If the cursor is local to a transaction, the lock is released when the transaction ends. If the cursor is declared WITH HOLD, the lock is released when the cursor is closed.

Informix provides the SET LOCK MODE instruction to define the lock wait timeout:

SET LOCK MODE TO { WAIT | NOT WAIT | WAIT seconds }

The default mode is NOT WAIT.

Dameng®

Dameng locks are released when closing the cursor or when the transaction ends.

Dameng's locking granularity is at the row level.0

The NOWAIT keyword can be used in SELECT ... FOR UPDATE statement, the return immediately if the row is already locked by another user:

SELECT ... FOR UPDATE [ OF col-list ] NOWAIT

Solution

Important:

Cursors declared with SELECT ... FOR UPDATE using the WITH HOLD clause cannot be supported with Dameng.

The database interface is based on an emulation of an Informix engine using transaction logging.

Opening a SELECT ... FOR UPDATE cursor declared outside a transaction will raise an SQL error -255 (not in transaction).

When using pessimistic locking with DECLARE ... CURSOR FOR SELECT ... FOR UPDATE, review the program logic to have OPEN and CLOSE instructions inside transactions (BEGIN WORK + COMMIT WORK / ROLLBACK WORK).