Provided by: postgresql-client-8.4_8.4.11-1_i386
LOCK - lock a table
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
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
Serializable isolation level, you have to execute the LOCK TABLE
statement before executing any SELECT or data modification statement.
A 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
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 in the documentation.
name The name (optionally schema-qualified) of an existing table to
lock. If ONLY is specified, only that table is locked. If ONLY
is not specified, the table and all its descendant tables (if
any) are locked.
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.
The lock mode specifies which locks this lock conflicts with.
Lock modes are described in 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
LOCK TABLE ... IN ACCESS SHARE MODE requires SELECT privileges on the
target table. All other forms of LOCK require at least one of 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 [begin(7)] and COMMIT [commit(7)] (or ROLLBACK
[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 in the documentation and the FOR UPDATE/FOR SHARE
Clause [select(7)] in the SELECT reference documentation.
Obtain a SHARE lock on a primary key table when going to perform
inserts into a foreign key table:
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!');
Take a SHARE ROW EXCLUSIVE lock on a primary key table when going to
perform a delete operation:
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;
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
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.