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9.3. Explicit Locking

PostgreSQL provides various lock modes to control concurrent access to data in tables. These modes can be used for application-controlled locking in situations where MVCC does not give the desired behavior. Also, most PostgreSQL commands automatically acquire locks of appropriate modes to ensure that referenced tables are not dropped or modified in incompatible ways while the command executes. (For example, ALTER TABLE cannot be executed concurrently with other operations on the same table.)

9.3.1. Table-Level Locks

The list below shows the available lock modes and the contexts in which they are used automatically by PostgreSQL. Remember that all of these lock modes are table-level locks, even if the name contains the word "row". The names of the lock modes are historical. To some extent the names reflect the typical usage of each lock mode --- but the semantics are all the same. The only real difference between one lock mode and another is the set of lock modes with which each conflicts. Two transactions cannot hold locks of conflicting modes on the same table at the same time. (However, a transaction never conflicts with itself --- for example, it may acquire ACCESS EXCLUSIVE lock and later acquire ACCESS SHARE lock on the same table.) Non-conflicting lock modes may be held concurrently by many transactions. Notice in particular that some lock modes are self-conflicting (for example, ACCESS EXCLUSIVE cannot be held by more than one transaction at a time) while others are not self-conflicting (for example, ACCESS SHARE can be held by multiple transactions). Once acquired, a lock mode is held till end of transaction.

To examine a list of the currently outstanding locks in a database server, use the pg_locks system view. For more information on monitoring the status of the lock manager subsystem, refer to the PostgreSQL 7.3.21 Administrator's Guide.

Table-level lock modes

ACCESS SHARE

Conflicts with the ACCESS EXCLUSIVE lock mode only.

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

Conflicts with the EXCLUSIVE and ACCESS EXCLUSIVE lock modes.

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

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

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

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

Acquired by VACUUM (without FULL).

SHARE

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

Acquired by CREATE INDEX.

SHARE ROW EXCLUSIVE

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

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

EXCLUSIVE

Conflicts with the ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE, and ACCESS EXCLUSIVE lock 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.

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

ACCESS EXCLUSIVE

Conflicts with locks of all modes (ACCESS SHARE, ROW SHARE, ROW EXCLUSIVE, SHARE UPDATE EXCLUSIVE, SHARE, SHARE ROW EXCLUSIVE, EXCLUSIVE, and ACCESS EXCLUSIVE). This mode guarantees that the holder is the only transaction accessing the table in any way.

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

Note: Only an ACCESS EXCLUSIVE lock blocks a SELECT (without FOR UPDATE) statement.

9.3.2. Row-Level Locks

In addition to table-level locks, there are row-level locks. A row-level lock on a specific row is automatically acquired when the row is updated (or deleted or marked for update). The lock is held until the transaction commits or rolls back. Row-level locks don't affect data querying; they block writers to the same row only. To acquire a row-level lock on a row without actually modifying the row, select the row with SELECT FOR UPDATE. Note that once a particular row-level lock is acquired, the transaction may update the row multiple times without fear of conflicts.

PostgreSQL doesn't remember any information about modified rows in memory, so it has no limit to the number of rows locked at one time. However, locking a row may cause a disk write; thus, for example, SELECT FOR UPDATE will modify selected rows to mark them and so will result in disk writes.

In addition to table and row locks, page-level share/exclusive locks are used to control read/write access to table pages in the shared buffer pool. These locks are released immediately after a tuple is fetched or updated. Application writers normally need not be concerned with page-level locks, but we mention them for completeness.

9.3.3. Deadlocks

Use of explicit locking can cause deadlocks, wherein two (or more) transactions each hold locks that the other wants. For example, if transaction 1 acquires an exclusive lock on table A and then tries to acquire an exclusive lock on table B, while transaction 2 has already exclusive-locked table B and now wants an exclusive lock on table A, then neither one can proceed. PostgreSQL automatically detects deadlock situations and resolves them by aborting one of the transactions involved, allowing the other(s) to complete. (Exactly which transaction will be aborted is difficult to predict and should not be relied on.)

The best defense against deadlocks is generally to avoid them by being certain that all applications using a database acquire locks on multiple objects in a consistent order. One should also ensure that the first lock acquired on an object in a transaction is the highest mode that will be needed for that object. If it is not feasible to verify this in advance, then deadlocks may be handled on-the-fly by retrying transactions that are aborted due to deadlock.

So long as no deadlock situation is detected, a transaction seeking either a table-level or row-level lock will wait indefinitely for conflicting locks to be released. This means it is a bad idea for applications to hold transactions open for long periods of time (e.g., while waiting for user input).

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