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.

ORACLE

ORACLE allows individual and exclusive row locking with:

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

A lock is acquired for each selected row when the cursor is opened, before the first fetch.

Cursors using SELECT ... FOR UPDATE are automatically closed when the transaction ends.

Locks are not released when a cursor is closed.

ORACLE's locking granularity is at the row level.

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 ORACLE.

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).

See also Cursors with Hold and UPDATE/DELETE WHERE CURRENT OF for more details.