LOCK(7) 						  PostgreSQL 9.2.7 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 9.2.7						    2014-02-17								   LOCK(7)