Oracle: Locking 101

Locking data is essential to prevent more than one user altering data in the database at the same time. Oracle can lock one row, multiple rows, or the whole table. Oracle attempts to lock at the lowest level possible to ensure minimal impact on the database. It is possible using LOCK TABLE to lock manually lock a complete table. I have personally never needed to use this statement and it is certainly not recommended for most applications with more than one concurrent user.

Querying the database will *never* produce a lock using Oracle, and unlike some databases locking will not stop Oracle producing consistent queries, as the query works from the UNDO tablespace. This is the technology that allows a long running query started at 1200 to finish at 1400 and show the results of that query with all the data as it was when the query was started at 1200, regardless of changes to the data during the time the query was running.

The first user to request a lock, gets the lock and other users are queued on a FIFO basis. If the design of your application requires that control is returned to the user immediately in the event of the requested data being unavailable for locking then the NOWAIT parameter can be used.

It is important to realise that all locks are released when the transaction is terminated via either a COMMIT or ROLLBACK, whether issued explicitly or implicitly.

The table below shows the different kinds of table lock modes available:

Row Share
Allows connect access to the whole table, stops users locking the entire table for EXCLUSIVE.

Row Exclusive
As Row Share, but stops other users locking in SHARE mode
(used by DML such as INSERT, UPDATE and DELETE)

Share
Permits concurrent queries but prevents any updates to the table.
(used by CREATE INDEX)

Share Row Exclusive
To query the whole table, and allow others to do so, but prevent them locking the table in SHARE mode or doing any updates.

Exclusive
Permits queries, prevents any DML.
(used by DROP TABLE)

To monitor the current locks in Oracle you can use the Enterprise Manager or alternatively query the following views:

V$SESSION
V$TRANSACTION
V$LOCK
V$LOCKED_OBJECT

You can obtain explicit locks on individual rows using the select ... for update statement:

select * from ords_data where ords_ref = ‘SJC001’ for update;

If somebody else attempts to get a lock on that row their session will either wait or if using the NOWAIT parameter control will be returned to them immediately with an Oracle Error: ORA-00054. You can trap for this within your application and bring up a message explaining that the record is locked and to try again later.

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