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.

Microsoft™ SQL Server

Microsoft SQL Server allows individual and exclusive row locking by using server cursors with Scroll Locks option, combined with SQL hints such as UPDLOCK in the FROM clause::

SELECT ... FROM tab1 WITH(UPDLOCK) WHERE ...

Note: In Transact-SQL, the FOR UPDATE [OF col-list] clause is not part of the SELECT syntax: It is part of T-SQL DECLARE CURSOR syntax.

Individual locks are acquired when fetching the rows.

When the cursor is opened outside a transaction (BDL WITH HOLD cursor option), locks are released when the cursor is closed.

When the cursor is opened inside a transaction, locks are released when the transaction ends.

SQL Server's locking granularity is at the row level, page level or table level (the level is automatically selected by the engine for optimization).

Solution

When executing a SELECT … FOR UPDATE in the program, the SQL Server database drivers remove the FOR UPDATE clause from the SQL text and set the ODBC cursor attribute SQL_ATTR_CONCURRENCY to SQL_CONCUR_LOCK. This enables Scroll Locks concurrency in the server cursor.

The Scroll Locks option implements pessimistic concurrency control, in which the application attempts to lock the underlying database rows at the time they are read into the cursor result set.

SQL Server Transact-SQL hints such as UPDLOCK can be used to fine-tune the locking semantics that will be used by SQL Server.

When using server cursors with Scroll Locks option, an update lock is placed on the row when it is read into the cursor. If the cursor is opened within a transaction, the transaction update lock is held until the transaction is either committed or rolled back; the cursor lock is dropped when the next row is fetched.

If the cursor has been opened outside a transaction, the lock is dropped when the next row is fetched.

Therefore, it is recommended that a cursor is opened in a transaction whenever the user wants full pessimistic concurrency control.

An update lock prevents any other task from acquiring an update or exclusive lock, which prevents any other task from updating the row.

An update lock, however, does not block a shared lock, so it does not prevent other tasks from reading the row unless the second task is also requesting a read with an update lock.

SELECT FOR UPDATE statements are well supported in BDL as long as they are used inside a transaction. Avoid cursors declared WITH HOLD.

Note: 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).

The SELECT FOR UPDATE statement cannot contain an ORDER BY clause if you want to perform positioned updates/deletes with WHERE CURRENT OF.

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

Review the program logic if you use pessimistic locking, because it is based on the NOT WAIT mode which is not supported by SQL Server.