PostgreSQL 9.2beta3 Documentation
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# LOCK

## Name

LOCK -- lock a table

## Synopsis

    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

## Description

`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 `REPEATABLE READ` or `SERIALIZABLE` isolation level, you have to execute the `LOCK TABLE` statement before executing any `SELECT` or data modification statement. A `REPEATABLE READ` or `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 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.) 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 Section 13.3.

## Parameters

`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.

`lockmode`

  The lock mode specifies which locks this lock conflicts with. Lock modes are described in Section 13.3.

  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 aborted.

## Notes

`LOCK TABLE ... IN ACCESS SHARE MODE` requires `SELECT` privileges on the target table. All other forms of `LOCK` require table-level `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 and COMMIT (or ROLLBACK) 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 Section 13.3.2 and the FOR UPDATE/FOR SHARE Clause in the `SELECT` reference documentation.

## 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 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.

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