LOCK(7) 							   SQL Commands 							   LOCK(7)

NAME

       LOCK - explicitly lock a table

SYNOPSIS

       LOCK [ TABLE ] name [, ...]
       LOCK [ TABLE ] name [, ...] IN lockmode MODE

       where lockmode is one of:

	    ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE |
	    SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE

   INPUTS
       name   The name (optionally schema-qualified) of an existing table to lock.

       ACCESS SHARE MODE
	      This is the least restrictive lock mode. It conflicts only with ACCESS EXCLUSIVE mode. It is used to protect a table from being mod-
	      ified by concurrent ALTER TABLE, DROP TABLE and VACUUM FULL commands.

	      Note: The SELECT command acquires a lock of this mode on referenced tables. In general, any query that only reads a table  and  does
	      not modify it will acquire this lock mode.

       ROW SHARE MODE
	      Conflicts with EXCLUSIVE and ACCESS EXCLUSIVE lock modes.

	      Note:  The  SELECT  FOR UPDATE command acquires a lock of this mode on the target table(s) (in addition to ACCESS SHARE locks on any
	      other tables that are referenced but not selected FOR UPDATE).

       ROW EXCLUSIVE MODE
	      Conflicts with SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes.

	      Note: The commands UPDATE, DELETE, and INSERT acquire this lock mode on the target table (in addition to ACCESS SHARE locks  on  any
	      other referenced tables). In general, this lock mode will be acquired by any query that modifies the data in a table.

       SHARE UPDATE EXCLUSIVE MODE
	      Conflicts  with SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode protects a table
	      against concurrent schema changes and VACUUM runs.

	      Note: Acquired by VACUUM (without FULL).

       SHARE MODE
	      Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode  protects
	      a table against concurrent data changes.

	      Note: Acquired by CREATE INDEX.

       SHARE ROW EXCLUSIVE MODE
	      Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes.

	      Note: This lock mode is not automatically acquired by any PostgreSQL command.

       EXCLUSIVE MODE
	      Conflicts  with  ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes.
	      This mode allows only concurrent ACCESS SHARE, i.e., only reads from the table can proceed in parallel with  a  transaction  holding
	      this lock mode.

	      Note: This lock mode is not automatically acquired by any PostgreSQL command.

       ACCESS EXCLUSIVE MODE
	      Conflicts with all lock modes. This mode guarantees that the holder is the only transaction accessing the table in any way.

	      Note: Acquired by ALTER TABLE, DROP TABLE, and VACUUM FULL statements.  This is also the default lock mode for LOCK TABLE statements
	      that do not specify a mode explicitly.

   OUTPUTS
       LOCK TABLE
	      The lock was successfully acquired.

       ERROR name: Table does not exist.
	      Message returned if name does not exist.

DESCRIPTION

       LOCK TABLE obtains a table-level lock, waiting if necessary for any conflicting locks to be released. 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 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 pre-
       vent concurrent data changes and ensure subsequent reads of the table see a stable view of committed data, because  SHARE  lock	mode  con-
       flicts  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 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.

	      Note: 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 DML statement. A serializable transaction's view of data will be frozen when its first DML  statement
	      begins. A later LOCK 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.)

       Two general rules may be followed to prevent deadlock conditions:

       o Transactions have to acquire locks on the same objects in the same order.

	 For example, if one application updates row R1 and than updates row R2 (in the same transaction) then the  second  application  shouldn't
	 update row R2 if it's going to update row R1 later (in a single transaction).	Instead, it should update rows R1 and R2 in the same order
	 as the first application.

       o If multiple lock modes are involved for a single object, then transactions should always acquire the most restrictive mode first.

	 An example for this rule was given previously when discussing the use of SHARE ROW EXCLUSIVE mode rather than SHARE mode.

       PostgreSQL does detect deadlocks and will rollback at least one waiting transaction to resolve the deadlock.  If it  is	not  practical	to
       code  an  application  to  follow  the  above rules strictly, an alternative solution is to be prepared to retry transactions when they are
       aborted by deadlocks.

       When locking multiple tables, the command LOCK a, b; is equivalent to LOCK a; LOCK b;. The tables are locked one-by-one in the order speci-
       fied in the LOCK command.

   NOTES
       LOCK  ...  IN ACCESS SHARE MODE requires SELECT privileges on the target table. All other forms of LOCK require UPDATE and/or DELETE privi-
       leges.

       LOCK is useful only inside a transaction block (BEGIN...COMMIT), since the lock is dropped as soon as the transaction ends. A LOCK  command
       appearing outside any transaction block forms a self-contained transaction, so the lock will be dropped as soon as it is obtained.

       RDBMS locking uses the following standard terminology:

       EXCLUSIVE
	      An exclusive lock prevents other locks of the same type from being granted.

       SHARE  A shared lock allows others to also hold the same type of lock, but prevents the corresponding EXCLUSIVE lock from being granted.

       ACCESS Locks table schema.

       ROW    Locks individual rows.

       PostgreSQL  does not follow this terminology exactly. 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 does not follow this naming convention accurately, since it 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.

USAGE

       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

   SQL92
       There is no LOCK TABLE in SQL92, which instead uses SET TRANSACTION to specify concurrency levels on transactions. We support 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(TM).

SQL - Language Statements					    2002-11-22								   LOCK(7)