SELECT FOR UPDATE

Many BDL programs implement pessimistic locking in order to prevent several users from editing the same rows at the same time.

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.

Genero db allows individual and exclusive row locking with:

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

• A lock is acquired for each selected row when the cursor is opened (before the first fetch).
• The lock is only released when the transaction ends.
• FOR UPDATE cursors can only be OPENed inside a transaction.

Genero db locking granularity is at the row level.

To control the behavior of the program when locking rows, Informix^® provides a specific instruction to set the wait mode:

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

The default mode is NOT WAIT. This is an Informix-specific SQL statement.