lock
LOCK() SQL Commands LOCK()
NAME
LOCK - lock a table
SYNOPSIS
LOCK [ TABLE ] name [, ...] [ IN lockmode MODE ]
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. 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 isola-
tion level read committed 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; fur-
thermore none can begin until you release the lock.
To achieve a similar effect when running a transaction at the isola-
tion level serializable, you have to execute the LOCK TABLE statement
before executing any data modification statement. A serializable
transaction’s view of data will be frozen when its first data modifi-
cation statement begins. A later LOCK TABLE will still prevent concur-
rent 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 ta-
ble, 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 EXCLU-
SIVE mode to actually perform their updates. (Note that a transac-
tion’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 [XRef to EXPLICIT-LOCKING].
PARAMETERS
name The name (optionally schema-qualified) of an existing table to
lock.
The command LOCK a, b; is equivalent to LOCK a; LOCK b;. The
tables are locked one-by-one in the order specified in the LOCK
command.
lockmode
The lock mode specifies which locks this lock conflicts with.
Lock modes are described in [XRef to EXPLICIT-LOCKING].
If no lock mode is specified, then ACCESS EXCLUSIVE, the most
restrictive mode, is used.
NOTES
LOCK ... IN ACCESS SHARE MODE requires SELECT privileges on the target
table. All other forms of LOCK require UPDATE and/or DELETE privi-
leges.
LOCK is useful only inside a transaction block (BEGIN/COMMIT pair),
since the lock is dropped as soon as the transaction ends. A LOCK com-
mand appearing outside any transaction block forms a self-contained
transaction, so the lock will be dropped as soon as it is obtained.
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 seman-
tics so far as LOCK TABLE is concerned, differing only in the rules
about which modes conflict with which.
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.
SQL - Language Statements 2008-01-03 LOCK()
