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LOCK

Name

LOCK — Explicit lock of a table inside a transaction
LOCK [ TABLE ] table
LOCK [ TABLE ] table IN [ ROW | ACCESS ] { SHARE | EXCLUSIVE } MODE
LOCK [ TABLE ] table IN SHARE ROW EXCLUSIVE MODE
  

Inputs

table

The name of an existing table to lock.

ACCESS SHARE MODE

Note: This lock mode is acquired automatically over tables being queried. Postgres releases automatically acquired ACCESS SHARE locks after the statement is done.

This is the least restrictive lock mode which conflicts only with ACCESS EXCLUSIVE mode. It is intended to protect a table being queried from concurrent ALTER TABLE, DROP TABLE and VACUUM statements over the same table.

ROW SHARE MODE

Note: Automatically acquired by any SELECT FOR UPDATE statement.

Conflicts with EXCLUSIVE and ACCESS EXCLUSIVE lock modes.

ROW EXCLUSIVE MODE

Note: Automatically acquired by any UPDATE, DELETE, INSERT statement.

Conflicts with SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes. Generally means that a transaction updated or inserted some tuples in a table.

SHARE MODE

Note: Automatically acquired by any CREATE INDEX statement.

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

SHARE ROW EXCLUSIVE MODE

Conflicts with ROW EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode is more restrictive than SHARE mode because of only one transaction at time can hold this lock.

EXCLUSIVE MODE

Conflicts with ROW SHARE, ROW EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE and ACCESS EXCLUSIVE modes. This mode is yet more restrictive than that of SHARE ROW EXCLUSIVE; it blocks all concurrent SELECT FOR UPDATE queries.

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 table: Table does not exist.

Message returned if table does not exist.

Description

Postgres always uses the least restrictive lock mode whenever possible. LOCK TABLE provided for cases when you might need more restrictive locking.

For example, an application runs a transaction at READ COMMITTED isolation level and needs to ensure the existance 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 table 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 execution 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 concurrent SHARE lock.

To continue with the deadlock (when two transaction wait 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 level on transactions. We support that too; see SET for details.

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