PostgreSQL 7.1.3 Documentation
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# LOCK

## Name

LOCK -- Explicitly lock a table inside a transaction

## Synopsis

    LOCK [ TABLE ] name
    LOCK [ TABLE ] name IN [ ROW | ACCESS ] { SHARE | EXCLUSIVE } MODE
    LOCK [ TABLE ] name IN SHARE ROW EXCLUSIVE MODE

### Inputs

`name`

  The name of an existing table to lock.

ACCESS SHARE MODE

  > Note: This lock mode is acquired automatically over tables being queried.

  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 commands.

ROW SHARE MODE

  > Note: Automatically acquired by SELECT...FOR UPDATE. While it is a shared lock, may be upgraded later to a ROW EXCLUSIVE lock.

  Conflicts with EXCLUSIVE and ACCESS EXCLUSIVE lock modes.

ROW EXCLUSIVE MODE

  > Note: Automatically acquired by UPDATE, DELETE, and INSERT statements.

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

SHARE MODE

  > Note: Automatically acquired by CREATE INDEX. Share-locks the entire table.

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

SHARE ROW EXCLUSIVE MODE

  > Note: This is like EXCLUSIVE MODE, but allows SHARE ROW locks by others.

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

EXCLUSIVE MODE

  > Note: This mode is yet more restrictive than SHARE ROW EXCLUSIVE. It blocks all concurrent ROW SHARE/SELECT...FOR UPDATE queries.

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

ACCESS EXCLUSIVE MODE

  > Note: Automatically acquired by ALTER TABLE, DROP TABLE, VACUUM statements. This is the most restrictive lock mode which conflicts with all other lock modes and protects a locked table from any concurrent operations.

  > Note: This lock mode is also acquired by an unqualified LOCK TABLE (i.e., the command without an explicit lock mode option).

### Outputs

`LOCK TABLE`

  The lock was successfully applied.

`ERROR `name`: Table does not exist.`

  Message returned if `name` does not exist.

## Description

LOCK TABLE controls concurrent access to a table for the duration of a transaction. Postgres always uses the least restrictive lock mode whenever possible. LOCK TABLE provides for cases when you might need more restrictive locking.

RDBMS locking uses the following terminology:

EXCLUSIVE

  Exclusive lock that prevents other locks from being granted.

SHARE

  Allows others to share lock. Prevents EXCLUSIVE locks.

ACCESS

  Locks table schema.

ROW

  Locks individual rows.

> Note: If EXCLUSIVE or SHARE are not specified, EXCLUSIVE is assumed. Locks exist for the duration of the transaction.

For example, an application runs a transaction at READ COMMITTED isolation level and needs to ensure the existence of data in a table for the duration of the transaction. To achieve this you could use SHARE lock mode over the table before querying. This will protect data from concurrent changes and provide any further read operations over the table with data in their actual current state, because SHARE lock mode conflicts with any ROW EXCLUSIVE one acquired by writers, and your LOCK TABLE `name` IN SHARE MODE statement will wait until any concurrent write operations commit or rollback.

> Note: To read data in their real current state when running a transaction at the SERIALIZABLE isolation level you have to execute a LOCK TABLE statement before executing any DML statement, when the transaction defines what concurrent changes will be visible to itself.

In addition to the requirements above, if a transaction is going to change data in a table, then SHARE ROW EXCLUSIVE lock mode should be acquired to prevent deadlock conditions when two concurrent transactions attempt to lock the table in SHARE mode and then try to change data in this table, both (implicitly) acquiring ROW EXCLUSIVE lock mode that conflicts with a concurrent SHARE lock.

To continue with the deadlock (when two transaction wait for one another) issue raised above, you should follow two general rules 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.

- Transactions should acquire two conflicting lock modes only if one of them is self-conflicting (i.e., may be held by one transaction at time only). If multiple lock modes are involved, 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.

> Note: Postgres does detect deadlocks and will rollback at least one waiting transaction to resolve the deadlock.

### Notes

LOCK is a Postgres language extension.

Except for ACCESS SHARE/EXCLUSIVE lock modes, all other Postgres lock modes and the LOCK TABLE syntax are compatible with those present in Oracle.

LOCK works only inside transactions.

## Usage

Illustrate 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.

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