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 speci-
       fied, 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 query-
       ing. 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  commit-
       ted 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 actu-
       ally perform their updates. (Note that a transaction's own locks never conflict, so a transaction can acquire ROW  EXCLU-
       SIVE  mode when it holds SHARE mode -- but not if anyone else holds SHARE mode.) To avoid deadlocks, make sure all trans-
       actions 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) can-
              not 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 [com-
       mit(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 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 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  transac-
       tions. 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                                  2011-09-22                                                    LOCK(7)