Name
LOCK -- lock a tableSynopsis
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
statement will wait until any concurrent holders of name
IN SHARE MODEROW 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. IfONLY
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 toLOCK TABLE a; LOCK TABLE b;
. The tables are locked one-by-one in the order specified in theLOCK 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 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 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 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.
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