|The PostgreSQL 9.0 Reference Manual - Volume 1B - SQL Command Reference
by The PostgreSQL Global Development Group
Paperback (6"x9"), 488 pages
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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,
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 acquiring locks automatically for commands that reference
tables, PostgreSQL always uses the least
restrictive lock mode possible.
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
ROW EXCLUSIVE lock acquired by writers, and your
LOCK TABLE name IN SHARE MODE
statement will wait until any concurrent holders of
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
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
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
mode, and then be unable to also acquire
mode to actually perform their updates. (Note that a transaction's
own locks never conflict, so a transaction can acquire
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 Volume 1A: 11.3 Explicit Locking.
The name (optionally schema-qualified) of an existing table to
ONLYis specified, only that table is locked. If
ONLYis 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
The lock mode specifies which locks this lock conflicts with.
Lock modes are described in Volume 1A: 11.3 Explicit Locking.
If no lock mode is specified, then
ACCESS EXCLUSIVE, the most restrictive mode, is used.
LOCK TABLEshould 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 MODE requires
privileges on the target table. All other forms of
require at least one of
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
is used outside a transaction block.
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 Volume 1A: 11.3.2 Row-Level Locks
and the FOR UPDATE/FOR SHARE Clause in the
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;
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;
There is no
LOCK TABLE in the SQL standard,
which instead uses
SET TRANSACTION to specify
concurrency levels on transactions. PostgreSQL supports that too;
SET TRANSACTION for details.
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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