LOCK(7) - man - phpman

LOCK(7)                            PostgreSQL 14.23 Documentation                            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 a view is locked, all relations appearing in the view definition query are also locked
       recursively with the same lock mode.

       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.

       The user performing the lock on the view must have the corresponding privilege on the view.
       In addition the view's owner must have the relevant privileges on the underlying base
       relations, but the user performing the lock does not need any permissions on the underlying
       base relations.

       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 and COMMIT (or ROLLBACK) 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 Locking
       Clause in the SELECT(7) 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.



PostgreSQL 14.23                                2026                                         LOCK(7)