LOCK - lock a table
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
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 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 Serializable isolation level, you have to execute the LOCK TABLE statement before executing any SELECT or data modification statement. A serializable transactions 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 wont 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 transactions 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 the documentation.
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 specified in the LOCK TABLE command.
lockmode The lock mode specifies which locks this lock conflicts with. Lock modes are described 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.
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 the documentation and the FOR UPDATE/FOR SHARE Clause [select(7)] in the SELECT reference documentation.
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;
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(7)] 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||LOCK ()||2010-12-14|