LOCK — lock a table
LOCK [ TABLE ] [ ONLY ]
name[ * ] [, ...] [ IN
lockmodeMODE ] [ NOWAIT ] where
lockmodeis one of: ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE | SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
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 transaction. (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 COMMITTED 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 prevent 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 statement will wait until any concurrent holders of
name IN SHARE MODE
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
REPEATABLE READ or
SERIALIZABLE isolation level, you have to execute the
LOCK TABLE statement before executing any
SELECT or data modification statement. A
REPEATABLE READ or
SERIALIZABLE transaction's view of data will be frozen when its first
SELECT or data modification statement begins. A
LOCK TABLE later in the transaction 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 Section 13.3.
The name (optionally schema-qualified) of an existing table to lock. If
ONLY is specified before the table name, only that table is locked. If
ONLY is not specified, the table and all its descendant tables (if any) are locked. Optionally,
* can be specified after the table name to explicitly indicate that descendant tables are included.
LOCK TABLE a, b; is equivalent to
LOCK TABLE a; LOCK TABLE b;. The tables are locked one-by-one in the order specified in the
LOCK TABLE command.
The lock mode specifies which locks this lock conflicts with. Lock modes are described in Section 13.3.
If no lock mode is specified, then
ACCESS EXCLUSIVE, the most restrictive mode, is used.
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.
LOCK TABLE ... IN ACCESS SHARE MODE requires
SELECT privileges on the target table.
LOCK TABLE ... IN ROW EXCLUSIVE MODE requires
TRUNCATE privileges on the target table. All other forms of
LOCK require table-level
LOCK TABLE is useless outside a transaction block: the lock would remain held only to the completion of the statement. Therefore PostgreSQL reports an error if
LOCK is used outside a transaction block. Use BEGIN and COMMIT (or ROLLBACK) to define a transaction block.
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 shareable 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 Section 13.3.2 and the The Locking Clause in the
SELECT reference documentation.
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;
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;
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 for details.
ACCESS EXCLUSIVE, and
SHARE UPDATE EXCLUSIVE lock modes, the PostgreSQL lock modes and the
LOCK TABLE syntax are compatible with those present in Oracle.
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