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LOCK(5)
NAME
  LOCK - lock a table

SYNOPSIS
  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

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 the isolation level
  read committed 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 isolation
  level serializable, you have to execute the LOCK TABLE statement before
  executing any data modification statement. A serializable transaction's
  view of data will be frozen when its first data modification statement
  begins. A later LOCK TABLE 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 the section called ``Explicit Locking'' in the documentation.

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

       The command LOCK a, b; is equivalent to LOCK a; LOCK b;. The tables
       are locked one-by-one in the order specified in the LOCK command.

  lockmode
       The lock mode specifies which locks this lock conflicts with.  Lock
       modes are described in the section called ``Explicit Locking'' in the
       documentation.

       If no lock mode is specified, then ACCESS EXCLUSIVE, the most
       restrictive mode, is used.

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

  LOCK is useful only inside a transaction block (BEGIN/COMMIT pair), 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.

  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.

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(5)] 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.

Index for Section 5

Alphabetical listing for L

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