| SQL adaptation guide For PostgreSQL 8.x.y, 9.x.y / BDL programming | |
A lot of BDL programs use pessimistic locking in order to avoid several users 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
In both Informix® and PostgreSQL, locks are released when closing the cursor or when the transaction ends.
PostgreSQL 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 as an Informix-specific SQL statement; PostgreSQL has no equivalent for "SET LOCK MODE TO NOT WAIT".
The database interface is based on an emulation of an Informix engine using transaction logging. Therefore, opening a SELECT ... FOR UPDATE cursor declared outside a transaction will raise an SQL error -255 (not in transaction).
You must review the program logic if you use pessimistic locking because it is based on the NOT WAIT mode which is not supported by PostgreSQL.