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LOCK

Name

LOCK  --  explicitly lock a table

Synopsis

LOCK [ TABLE ] name [, ...]
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
  

Inputs

name

The name (optionally schema-qualified) of an existing table to lock.

ACCESS SHARE MODE

This is the least restrictive lock mode. It conflicts only with ACCESS EXCLUSIVE mode. It is used to protect a table from being modified by concurrent ALTER TABLE, DROP TABLE and VACUUM FULL commands.

Note: The SELECT command acquires a lock of this mode on referenced tables. In general, any query that only reads a table and does not modify it will acquire this lock mode.

ROW SHARE MODE

Conflicts with EXCLUSIVE and ACCESS EXCLUSIVE lock modes.

Note: The SELECT FOR UPDATE command acquires a lock of this mode on the target table(s) (in addition to ACCESS SHARE locks on any other tables that are referenced but not selected FOR UPDATE).

ROW EXCLUSIVE MODE

Conflicts with SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes.

Note: The commands UPDATE, DELETE, and INSERT acquire this lock mode on the target table (in addition to ACCESS SHARE locks on any other referenced tables). In general, this lock mode will be acquired by any query that modifies the data in a table.

SHARE UPDATE EXCLUSIVE MODE

Conflicts with SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode protects a table against concurrent schema changes and VACUUM runs.

Note: Acquired by VACUUM (without FULL).

SHARE MODE

Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode protects a table against concurrent data changes.

Note: Acquired by CREATE INDEX.

SHARE ROW EXCLUSIVE MODE

Conflicts with ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes.

Note: This lock mode is not automatically acquired by any PostgreSQL command.

EXCLUSIVE MODE

Conflicts with ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode allows only concurrent ACCESS SHARE, i.e., only reads from the table can proceed in parallel with a transaction holding this lock mode.

Note: This lock mode is not automatically acquired by any PostgreSQL command.

ACCESS EXCLUSIVE MODE

Conflicts with all lock modes. This mode guarantees that the holder is the only transaction accessing the table in any way.

Note: Acquired by ALTER TABLE, DROP TABLE, and VACUUM FULL statements. This is also the default lock mode for LOCK TABLE statements that do not specify a mode explicitly.

Outputs

LOCK TABLE

The lock was successfully acquired.

ERROR name: Table does not exist.

Message returned if name does not exist.

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 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 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.

Note: 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 DML statement. A serializable transaction's view of data will be frozen when its first DML statement begins. A later LOCK 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 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.)

Two general rules may be followed to prevent deadlock conditions:

  • Transactions have to acquire locks on the same objects in the same order.

    For example, if one application updates row R1 and than updates row R2 (in the same transaction) then the second application shouldn't update row R2 if it's going to update row R1 later (in a single transaction). Instead, it should update rows R1 and R2 in the same order as the first application.

  • If multiple lock modes are involved for a single object, then transactions should always acquire the most restrictive mode first.

    An example for this rule was given previously when discussing the use of SHARE ROW EXCLUSIVE mode rather than SHARE mode.

PostgreSQL does detect deadlocks and will rollback at least one waiting transaction to resolve the deadlock. If it is not practical to code an application to follow the above rules strictly, an alternative solution is to be prepared to retry transactions when they are aborted by deadlocks.

When locking multiple tables, 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.

Notes

LOCK ... IN ACCESS SHARE MODE requires SELECT privileges on the target table. All other forms of LOCK require UPDATE and/or DELETE privileges.

LOCK is useful only inside a transaction block (BEGIN...COMMIT), since the lock is dropped as soon as the transaction ends. A LOCK command appearing outside any transaction block forms a self-contained transaction, so the lock will be dropped as soon as it is obtained.

RDBMS locking uses the following standard terminology:

EXCLUSIVE

An exclusive lock prevents other locks of the same type from being granted.

SHARE

A shared lock allows others to also hold the same type of lock, but prevents the corresponding EXCLUSIVE lock from being granted.

ACCESS

Locks table schema.

ROW

Locks individual rows.

PostgreSQL does not follow this terminology exactly. 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 does not follow this naming convention accurately, since it 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.

Usage

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

SQL92

There is no LOCK TABLE in SQL92, which instead uses SET TRANSACTION to specify concurrency levels on transactions. We support that too; see SET TRANSACTION 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(TM).

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