LOCK — lock a table
LOCK [ TABLE ] [ ONLY ]
name[ * ] [, ...] [ IN
lockmodeMODE ] [ NOWAIT ] where
lockmodeis one of: ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE | SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
LOCK TABLE obtains a
table-level lock, waiting if necessary for any conflicting
locks to be released. If
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
TABLE command; locks are always released at transaction
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
SHARE lock mode
conflicts with the
lock acquired by writers, and your
TABLE statement will wait until any concurrent holders of
name IN SHARE
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
REPEATABLE READ or
SERIALIZABLE isolation level, you
have to execute the
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
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
If a transaction of this sort is going to change the data in
the table, then it should use
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
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.
The name (optionally schema-qualified) of an existing
table to lock. If
specified before the table name, only that table is
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
LOCK TABLE a,
b; is equivalent to
TABLE a; LOCK TABLE b;. The tables are locked
one-by-one in the order specified in the
LOCK TABLE command.
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.
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.
LOCK TABLE ... IN ACCESS SHARE
privileges on the target table.
TABLE ... IN ROW EXCLUSIVE MODE requires
TRUNCATE privileges on the target table. All
other forms of
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
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
SHARE lock on a
primary key table when going to perform inserts into a foreign
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;
SHARE ROW EXCLUSIVE lock
on a primary key table when going to perform a delete
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;
There is no
LOCK TABLE in the
SQL standard, which instead uses
TRANSACTION to specify concurrency levels on
supports that too; see SET TRANSACTION for details.
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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