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LOCK

LOCK — lock a table

Synopsis

LOCK [ TABLE ] [ ONLY ]name [ * ] [, ...] [ INlockmode MODE ] [ NOWAIT ]wherelockmode 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. IfNOWAIT 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 noUNLOCK TABLE command; locks are always released at transaction end.)

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 theREAD 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 obtainSHARE 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, becauseSHARE lock mode conflicts with theROW EXCLUSIVE lock acquired by writers, and yourLOCK TABLEname IN SHARE MODE statement will wait until any concurrent holders ofROW 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 theREPEATABLE READ orSERIALIZABLE isolation level, you have to execute theLOCK TABLE statement before executing anySELECT or data modification statement. AREPEATABLE READ orSERIALIZABLE transaction's view of data will be frozen when its firstSELECT or data modification statement begins. ALOCK 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 useSHARE ROW EXCLUSIVE lock mode instead ofSHARE mode. This ensures that only one transaction of this type runs at a time. Without this, a deadlock is possible: two transactions might both acquireSHARE mode, and then be unable to also acquireROW EXCLUSIVE mode to actually perform their updates. (Note that a transaction's own locks never conflict, so a transaction can acquireROW EXCLUSIVE mode when it holdsSHARE mode — but not if anyone else holdsSHARE 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 inSection 13.3.

Parameters

Notes

LOCK TABLE ... IN ACCESS SHARE MODE requiresSELECT privileges on the target table.LOCK TABLE ... IN ROW EXCLUSIVE MODE requiresINSERT,UPDATE,DELETE, orTRUNCATE privileges on the target table. All other forms ofLOCK require table-levelUPDATE,DELETE, orTRUNCATE privileges.

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. ThereforePostgreSQL reports an error ifLOCK is used outside a transaction block. UseBEGIN andCOMMIT (orROLLBACK) to define a transaction block.

LOCK TABLE only deals with table-level locks, and so the mode names involvingROW 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 asLOCK 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, seeSection 13.3.2 andThe Locking Clause in theSELECT documentation.

Examples

Obtain aSHARE 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 returnedINSERT INTO films_user_comments VALUES    (_id_, 'GREAT! I was waiting for it for so long!');COMMIT WORK;

Take aSHARE 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 noLOCK TABLE in the SQL standard, which instead usesSET TRANSACTION to specify concurrency levels on transactions.PostgreSQL supports that too; seeSET TRANSACTION for details.

Except forACCESS SHARE,ACCESS EXCLUSIVE, andSHARE UPDATE EXCLUSIVE lock modes, thePostgreSQL lock modes and theLOCK TABLE syntax are compatible with those present inOracle.