LOCK() SQL Commands LOCK()
NAME
LOCK - lock a table
SYNOPSIS
LOCK [ TABLE ] name [, ...] [ IN lockmode MODE ] [ NOWAIT ]
where lockmode is one of:
ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
| SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
DESCRIPTION
LOCK TABLE obtains a table-level lock, waiting if necessary for any
conflicting locks to be released. If NOWAIT is specified, LOCK TABLE
does not wait to acquire the desired lock: if it cannot be acquired
immediately, the command is aborted and an error is emitted. Once
obtained, the lock is held for the remainder of the current transac-
tion. (There is no UNLOCK TABLE command; locks are always released at
transaction end.)
When acquiring locks automatically for commands that reference tables,
PostgreSQL always uses the least restrictive lock mode possible. LOCK
TABLE provides for cases when you might need more restrictive locking.
For example, suppose an application runs a transaction at the Read Com-
mitted isolation level and needs to ensure that data in a table remains
stable for the duration of the transaction. To achieve this you could
obtain SHARE lock mode over the table before querying. This will pre-
vent concurrent data changes and ensure subsequent reads of the table
see a stable view of committed data, because SHARE lock mode conflicts
with the ROW EXCLUSIVE lock acquired by writers, and your LOCK TABLE
name IN SHARE MODE statement will wait until any concurrent holders of
ROW EXCLUSIVE mode locks commit or roll back. Thus, once you obtain the
lock, there are no uncommitted writes outstanding; furthermore none can
begin until you release the lock.
To achieve a similar effect when running a transaction at the Serializ-
able isolation level, you have to execute the LOCK TABLE statement
before executing any SELECT or data modification statement. A serial-
izable transaction's view of data will be frozen when its first SELECT
or data modification statement begins. A LOCK TABLE later in the trans-
action will still prevent concurrent writes -- but it won't ensure that
what the transaction reads corresponds to the latest committed values.
If a transaction of this sort is going to change the data in the table,
then it should use SHARE ROW EXCLUSIVE lock mode instead of SHARE mode.
This ensures that only one transaction of this type runs at a time.
Without this, a deadlock is possible: two transactions might both
acquire SHARE mode, and then be unable to also acquire ROW EXCLUSIVE
mode to actually perform their updates. (Note that a transaction's own
locks never conflict, so a transaction can acquire ROW EXCLUSIVE mode
when it holds SHARE mode -- but not if anyone else holds SHARE mode.)
To avoid deadlocks, make sure all transactions acquire locks on the
same objects in the same order, and if multiple lock modes are involved
for a single object, then transactions should always acquire the most
restrictive mode first.
More information about the lock modes and locking strategies can be
found in in the documentation.
PARAMETERS
name The name (optionally schema-qualified) of an existing table to
lock.
The command LOCK TABLE a, b; is equivalent to LOCK TABLE a; LOCK
TABLE b;. The tables are locked one-by-one in the order speci-
fied in the LOCK TABLE command.
lockmode
The lock mode specifies which locks this lock conflicts with.
Lock modes are described in in the documentation.
If no lock mode is specified, then ACCESS EXCLUSIVE, the most
restrictive mode, is used.
NOWAIT Specifies that LOCK TABLE should not wait for any conflicting
locks to be released: if the specified lock(s) cannot be
acquired immediately without waiting, the transaction is
aborted.
NOTES
LOCK TABLE ... IN ACCESS SHARE MODE requires SELECT privileges on the
target table. All other forms of LOCK require UPDATE and/or DELETE
privileges.
LOCK TABLE is useful only inside a transaction block (BEGIN/COMMIT
pair), since the lock is dropped as soon as the transaction ends. A
LOCK TABLE command appearing outside any transaction block forms a
self-contained transaction, so the lock will be dropped as soon as it
is obtained.
LOCK TABLE only deals with table-level locks, and so the mode names
involving ROW are all misnomers. These mode names should generally be
read as indicating the intention of the user to acquire row-level locks
within the locked table. Also, ROW EXCLUSIVE mode is a sharable table
lock. Keep in mind that all the lock modes have identical semantics so
far as LOCK TABLE is concerned, differing only in the rules about which
modes conflict with which. For information on how to acquire an actual
row-level lock, see in the documentation and the FOR UPDATE/FOR SHARE
Clause [select(l)] in the SELECT reference documentation.
EXAMPLES
Obtain a SHARE lock on a primary key table when going to perform
inserts into a foreign key table:
BEGIN WORK;
LOCK TABLE films IN SHARE MODE;
SELECT id FROM films
WHERE name = 'Star Wars: Episode I - The Phantom Menace';
-- Do ROLLBACK if record was not returned
INSERT INTO films_user_comments VALUES
(_id_, 'GREAT! I was waiting for it for so long!');
COMMIT WORK;
Take a SHARE ROW EXCLUSIVE lock on a primary key table when going to
perform a delete operation:
BEGIN WORK;
LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE;
DELETE FROM films_user_comments WHERE id IN
(SELECT id FROM films WHERE rating < 5);
DELETE FROM films WHERE rating < 5;
COMMIT WORK;
COMPATIBILITY
There is no LOCK TABLE in the SQL standard, which instead uses SET
TRANSACTION to specify concurrency levels on transactions. PostgreSQL
supports that too; see SET TRANSACTION [set_transaction(l)] for
details.
Except for ACCESS SHARE, ACCESS EXCLUSIVE, and SHARE UPDATE EXCLUSIVE
lock modes, the PostgreSQL lock modes and the LOCK TABLE syntax are
compatible with those present in Oracle.
SQL - Language Statements 2008-09-19 LOCK()
postgresql 8.3.4 - Generated Sat Oct 4 06:25:51 CDT 2008