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.

Netezza®

With Netezza, locks are released when closing the cursor or when the transaction ends.

Netezza does not support SELECT FOR UPDATE statements: Netezza systems are designed for data warehouse applications, not for OLTP applications. In a DW context, concurrent data access is not required or a priority.

Solution

Review the program logic when using SELECT FOR UPDATE statements.