LOCK(7)			PostgreSQL 9.2.24 Documentation		       LOCK(7)



NAME
       LOCK - lock a table

SYNOPSIS
       LOCK [ TABLE ] [ ONLY ] 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
       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
       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, "Explicit Locking", in the documentation.

PARAMETERS
       name
	   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.

	   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 Section 13.3, "Explicit Locking", 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 table-level UPDATE,
       DELETE, or TRUNCATE privileges.

       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(7) and COMMIT(7) (or ROLLBACK(7)) 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 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 Section 13.3.2, "Row-level Locks", in the
       documentation and the FOR UPDATE/FOR SHARE Clause 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(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.



PostgreSQL 9.2.24		  2017-11-06			       LOCK(7)