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.

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.

           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.  LOCK
       TABLE ... IN ROW EXCLUSIVE MODE requires INSERT, UPDATE, DELETE, or TRUNCATE 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
       shareable 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 and
       the The Locking 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.