13.3. Explicit Locking
Postgres Pro 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 Postgres Pro 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, TRUNCATE
cannot safely be executed concurrently with other operations on the same table, so it obtains an ACCESS EXCLUSIVE
lock on the table to enforce that.)
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 Chapter 26.
13.3.1. Table-Level Locks
The list below shows the available lock modes and the contexts in which they are used automatically by Postgres Pro. You can also acquire any of these locks explicitly with the command LOCK. 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 (see Table 13.2). 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 might acquire ACCESS EXCLUSIVE
lock and later acquire ACCESS SHARE
lock on the same table.) Non-conflicting lock modes can be held concurrently by many transactions. Notice in particular that some lock modes are self-conflicting (for example, an ACCESS EXCLUSIVE
lock cannot be held by more than one transaction at a time) while others are not self-conflicting (for example, an ACCESS SHARE
lock can be held by multiple transactions).
Table-Level Lock Modes
-
ACCESS SHARE
(AccessShareLock
) 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
(RowShareLock
) Conflicts with the
EXCLUSIVE
andACCESS EXCLUSIVE
lock modes.The
SELECT FOR UPDATE
andSELECT FOR SHARE
commands acquire a lock of this mode on the target table(s) (in addition toACCESS SHARE
locks on any other tables that are referenced but not selectedFOR UPDATE/FOR SHARE
).-
ROW EXCLUSIVE
(RowExclusiveLock
) Conflicts with the
SHARE
,SHARE ROW EXCLUSIVE
,EXCLUSIVE
, andACCESS EXCLUSIVE
lock modes.The commands
UPDATE
,DELETE
, andINSERT
acquire this lock mode on the target table (in addition toACCESS SHARE
locks on any other referenced tables). In general, this lock mode will be acquired by any command that modifies data in a table.-
SHARE UPDATE EXCLUSIVE
(ShareUpdateExclusiveLock
) Conflicts with the
SHARE UPDATE EXCLUSIVE
,SHARE
,SHARE ROW EXCLUSIVE
,EXCLUSIVE
, andACCESS EXCLUSIVE
lock modes. This mode protects a table against concurrent schema changes andVACUUM
runs.Acquired by
VACUUM
(withoutFULL
),ANALYZE
,CREATE INDEX CONCURRENTLY
,CREATE STATISTICS
,COMMENT ON
,REINDEX CONCURRENTLY
, and certainALTER INDEX
andALTER TABLE
variants (for full details see ALTER INDEX and ALTER TABLE).-
SHARE
(ShareLock
) Conflicts with the
ROW EXCLUSIVE
,SHARE UPDATE EXCLUSIVE
,SHARE ROW EXCLUSIVE
,EXCLUSIVE
, andACCESS EXCLUSIVE
lock modes. This mode protects a table against concurrent data changes.Acquired by
CREATE INDEX
(withoutCONCURRENTLY
).-
SHARE ROW EXCLUSIVE
(ShareRowExclusiveLock
) Conflicts with the
ROW EXCLUSIVE
,SHARE UPDATE EXCLUSIVE
,SHARE
,SHARE ROW EXCLUSIVE
,EXCLUSIVE
, andACCESS EXCLUSIVE
lock modes. This mode protects a table against concurrent data changes, and is self-exclusive so that only one session can hold it at a time.Acquired by
CREATE TRIGGER
and some forms ofALTER TABLE
(see ALTER TABLE).-
EXCLUSIVE
(ExclusiveLock
) Conflicts with the
ROW SHARE
,ROW EXCLUSIVE
,SHARE UPDATE EXCLUSIVE
,SHARE
,SHARE ROW EXCLUSIVE
,EXCLUSIVE
, andACCESS EXCLUSIVE
lock modes. This mode allows only concurrentACCESS SHARE
locks, i.e., only reads from the table can proceed in parallel with a transaction holding this lock mode.Acquired by
REFRESH MATERIALIZED VIEW CONCURRENTLY
.-
ACCESS EXCLUSIVE
(AccessExclusiveLock
) Conflicts with locks of
ACCESS SHARE
,ROW SHARE
,ROW EXCLUSIVE
,SHARE UPDATE EXCLUSIVE
,SHARE
,SHARE ROW EXCLUSIVE
,EXCLUSIVE
, andACCESS EXCLUSIVE
mode. This mode guarantees that the holder is the only transaction accessing the table in any way.Acquired by the
DROP TABLE
,TRUNCATE
,REINDEX
,CLUSTER
,VACUUM FULL
, andREFRESH MATERIALIZED VIEW
(withoutCONCURRENTLY
) commands. Many forms ofALTER INDEX
andALTER TABLE
also acquire a lock at this level. This is also the default lock mode forLOCK TABLE
statements that do not specify a mode explicitly.
Tip
Only an ACCESS EXCLUSIVE
lock blocks a SELECT
(without FOR UPDATE/SHARE
) statement.
Besides, there are two additional lock modes designed to support 1C Enterprise. These modes do not conflict with any other lock modes described above. Their use is possible, but not encouraged, as advisory locks provide the same functionality.
-
APPLICATION SHARE
Conflicts with the
APPLICATION EXCLUSIVE
lock mode only.-
APPLICATION EXCLUSIVE
Conflicts with the
APPLICATION SHARE
andAPPLICATION EXCLUSIVE
lock modes.
Once acquired, a lock is normally held until the end of the transaction. But if a lock is acquired after establishing a savepoint, the lock is released immediately if the savepoint is rolled back to. This is consistent with the principle that ROLLBACK
cancels all effects of the commands since the savepoint. The same holds for locks acquired within a PL/pgSQL exception block: an error escape from the block releases locks acquired within it.
Table 13.2. Conflicting Lock Modes
Requested Lock Mode | Existing Lock Mode | |||||||||
---|---|---|---|---|---|---|---|---|---|---|
ACCESS SHARE | ROW SHARE | ROW EXCL. | SHARE UPDATE EXCL. | SHARE | SHARE ROW EXCL. | EXCL. | ACCESS EXCL. | APP. SHARE | APP. EXCL. | |
ACCESS SHARE | X | |||||||||
ROW SHARE | X | X | ||||||||
ROW EXCL. | X | X | X | X | ||||||
SHARE UPDATE EXCL. | X | X | X | X | X | |||||
SHARE | X | X | X | X | X | |||||
SHARE ROW EXCL. | X | X | X | X | X | X | ||||
EXCL. | X | X | X | X | X | X | X | |||
ACCESS EXCL. | X | X | X | X | X | X | X | X | ||
APP. SHARE | X | |||||||||
APP. EXCL. | X | X |
13.3.2. Row-Level Locks
In addition to table-level locks, there are row-level locks, which are listed as below with the contexts in which they are used automatically by Postgres Pro. See Table 13.3 for a complete table of row-level lock conflicts. Note that a transaction can hold conflicting locks on the same row, even in different subtransactions; but other than that, two transactions can never hold conflicting locks on the same row. Row-level locks do not affect data querying; they block only writers and lockers to the same row. Row-level locks are released at transaction end or during savepoint rollback, just like table-level locks.
Row-Level Lock Modes
-
FOR UPDATE
FOR UPDATE
causes the rows retrieved by theSELECT
statement to be locked as though for update. This prevents them from being locked, modified or deleted by other transactions until the current transaction ends. That is, other transactions that attemptUPDATE
,DELETE
,SELECT FOR UPDATE
,SELECT FOR NO KEY UPDATE
,SELECT FOR SHARE
orSELECT FOR KEY SHARE
of these rows will be blocked until the current transaction ends; conversely,SELECT FOR UPDATE
will wait for a concurrent transaction that has run any of those commands on the same row, and will then lock and return the updated row (or no row, if the row was deleted). Within aREPEATABLE READ
orSERIALIZABLE
transaction, however, an error will be thrown if a row to be locked has changed since the transaction started. For further discussion see Section 13.4.The
FOR UPDATE
lock mode is also acquired by anyDELETE
on a row, and also by anUPDATE
that modifies the values of certain columns. Currently, the set of columns considered for theUPDATE
case are those that have a unique index on them that can be used in a foreign key (so partial indexes and expressional indexes are not considered), but this may change in the future.-
FOR NO KEY UPDATE
Behaves similarly to
FOR UPDATE
, except that the lock acquired is weaker: this lock will not blockSELECT FOR KEY SHARE
commands that attempt to acquire a lock on the same rows. This lock mode is also acquired by anyUPDATE
that does not acquire aFOR UPDATE
lock.-
FOR SHARE
Behaves similarly to
FOR NO KEY UPDATE
, except that it acquires a shared lock rather than exclusive lock on each retrieved row. A shared lock blocks other transactions from performingUPDATE
,DELETE
,SELECT FOR UPDATE
orSELECT FOR NO KEY UPDATE
on these rows, but it does not prevent them from performingSELECT FOR SHARE
orSELECT FOR KEY SHARE
.-
FOR KEY SHARE
Behaves similarly to
FOR SHARE
, except that the lock is weaker:SELECT FOR UPDATE
is blocked, but notSELECT FOR NO KEY UPDATE
. A key-shared lock blocks other transactions from performingDELETE
or anyUPDATE
that changes the key values, but not otherUPDATE
, and neither does it preventSELECT FOR NO KEY UPDATE
,SELECT FOR SHARE
, orSELECT FOR KEY SHARE
.
Postgres Pro doesn't remember any information about modified rows in memory, so there is no limit on the number of rows locked at one time. However, locking a row might cause a disk write, e.g., SELECT FOR UPDATE
modifies selected rows to mark them locked, and so will result in disk writes.
Table 13.3. Conflicting Row-Level Locks
Requested Lock Mode | Current Lock Mode | |||
---|---|---|---|---|
FOR KEY SHARE | FOR SHARE | FOR NO KEY UPDATE | FOR UPDATE | |
FOR KEY SHARE | X | |||
FOR SHARE | X | X | ||
FOR NO KEY UPDATE | X | X | X | |
FOR UPDATE | X | X | X | X |
13.3.3. Page-Level Locks
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 row is fetched or updated. Application developers normally need not be concerned with page-level locks, but they are mentioned here for completeness.
13.3.4. Deadlocks
The use of explicit locking can increase the likelihood of 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. Postgres Pro 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 upon.)
Note that deadlocks can also occur as the result of row-level locks (and thus, they can occur even if explicit locking is not used). Consider the case in which two concurrent transactions modify a table. The first transaction executes:
UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 11111;
This acquires a row-level lock on the row with the specified account number. Then, the second transaction executes:
UPDATE accounts SET balance = balance + 100.00 WHERE acctnum = 22222; UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 11111;
The first UPDATE
statement successfully acquires a row-level lock on the specified row, so it succeeds in updating that row. However, the second UPDATE
statement finds that the row it is attempting to update has already been locked, so it waits for the transaction that acquired the lock to complete. Transaction two is now waiting on transaction one to complete before it continues execution. Now, transaction one executes:
UPDATE accounts SET balance = balance - 100.00 WHERE acctnum = 22222;
Transaction one attempts to acquire a row-level lock on the specified row, but it cannot: transaction two already holds such a lock. So it waits for transaction two to complete. Thus, transaction one is blocked on transaction two, and transaction two is blocked on transaction one: a deadlock condition. Postgres Pro will detect this situation and abort one of the transactions.
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. In the example above, if both transactions had updated the rows in the same order, no deadlock would have occurred. One should also ensure that the first lock acquired on an object in a transaction is the most restrictive mode that will be needed for that object. If it is not feasible to verify this in advance, then deadlocks can be handled on-the-fly by retrying transactions that abort due to deadlocks.
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).
13.3.5. Advisory Locks
Postgres Pro provides a means for creating locks that have application-defined meanings. These are called advisory locks, because the system does not enforce their use — it is up to the application to use them correctly. Advisory locks can be useful for locking strategies that are an awkward fit for the MVCC model. For example, a common use of advisory locks is to emulate pessimistic locking strategies typical of so-called “flat file” data management systems. While a flag stored in a table could be used for the same purpose, advisory locks are faster, avoid table bloat, and are automatically cleaned up by the server at the end of the session.
There are two ways to acquire an advisory lock in Postgres Pro: at session level or at transaction level. Once acquired at session level, an advisory lock is held until explicitly released or the session ends. Unlike standard lock requests, session-level advisory lock requests do not honor transaction semantics: a lock acquired during a transaction that is later rolled back will still be held following the rollback, and likewise an unlock is effective even if the calling transaction fails later. A lock can be acquired multiple times by its owning process; for each completed lock request there must be a corresponding unlock request before the lock is actually released. Transaction-level lock requests, on the other hand, behave more like regular lock requests: they are automatically released at the end of the transaction, and there is no explicit unlock operation. This behavior is often more convenient than the session-level behavior for short-term usage of an advisory lock. Session-level and transaction-level lock requests for the same advisory lock identifier will block each other in the expected way. If a session already holds a given advisory lock, additional requests by it will always succeed, even if other sessions are awaiting the lock; this statement is true regardless of whether the existing lock hold and new request are at session level or transaction level.
Like all locks in Postgres Pro, a complete list of advisory locks currently held by any session can be found in the pg_locks
system view.
Both advisory locks and regular locks are stored in a shared memory pool whose size is defined by the configuration variables max_locks_per_transaction and max_connections. Care must be taken not to exhaust this memory or the server will be unable to grant any locks at all. This imposes an upper limit on the number of advisory locks grantable by the server, typically in the tens to hundreds of thousands depending on how the server is configured.
In certain cases using advisory locking methods, especially in queries involving explicit ordering and LIMIT
clauses, care must be taken to control the locks acquired because of the order in which SQL expressions are evaluated. For example:
SELECT pg_advisory_lock(id) FROM foo WHERE id = 12345; -- ok SELECT pg_advisory_lock(id) FROM foo WHERE id > 12345 LIMIT 100; -- danger! SELECT pg_advisory_lock(q.id) FROM ( SELECT id FROM foo WHERE id > 12345 LIMIT 100 ) q; -- ok
In the above queries, the second form is dangerous because the LIMIT
is not guaranteed to be applied before the locking function is executed. This might cause some locks to be acquired that the application was not expecting, and hence would fail to release (until it ends the session). From the point of view of the application, such locks would be dangling, although still viewable in pg_locks
.
The functions provided to manipulate advisory locks are described in Section 9.27.10.