LOCK

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.

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