omp_test_lock(lock) - phpMan

LOCK(7) PostgreSQL 9.2.24 Documentation LOCK(7)

NAME
LOCK - lock a table

SYNOPSIS
LOCK [ TABLE ] [ ONLY ] name [ * ] [, ...] [ IN lockmode MODE ] [ NOWAIT ]

where lockmode is one of:

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,
"Explicit Locking", in the documentation.

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, "Explicit Locking", 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) cannot 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 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
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 Section 13.3.2,
"Row-level Locks", in the documentation and the FOR UPDATE/FOR SHARE 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.

PostgreSQL 9.2.24 2017-11-06 LOCK(7)

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