LOCK(7) 							   SQL Commands 							   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 Serializable isolation level, you have to execute the  LOCK  TABLE  statement
       before executing any SELECT or data modification statement.  A 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 in the documentation.

PARAMETERS

       name   The name (optionally schema-qualified) of an existing table to lock. If ONLY is specified, only that table is locked. If ONLY is not
	      specified, the table and all its descendant tables (if any) are locked.

	      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 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 at least one of 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 [begin(7)] and COMMIT [commit(7)] (or ROLLBACK [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 ta-
       ble 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(7)] 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.

SQL - Language Statements					    2010-05-14								   LOCK(7)