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, 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 27.
The list below shows the available lock modes and the contexts in
which they are used automatically by
PostgreSQL. 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 and
ACCESS EXCLUSIVE lock modes.
The SELECT command acquires a lock of this mode
on all tables on which one of the FOR UPDATE,
FOR NO KEY UPDATE,
FOR SHARE, or
FOR KEY SHARE options is specified
(in addition to ACCESS SHARE locks on any other
tables that are referenced without any explicit
FOR ... locking option).
ROW EXCLUSIVE (RowExclusiveLock)
Conflicts with the SHARE, SHARE ROW
EXCLUSIVE, EXCLUSIVE, and
ACCESS EXCLUSIVE lock modes.
The commands UPDATE,
DELETE, INSERT, and
MERGE
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
command that modifies data in a table.
SHARE UPDATE EXCLUSIVE (ShareUpdateExclusiveLock)
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),
ANALYZE, CREATE INDEX CONCURRENTLY,
CREATE STATISTICS, COMMENT ON,
REINDEX CONCURRENTLY,
and certain ALTER INDEX
and ALTER TABLE variants
(for full details see the documentation of these commands).
SHARE (ShareLock)
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
(without CONCURRENTLY).
SHARE ROW EXCLUSIVE (ShareRowExclusiveLock)
Conflicts with the ROW EXCLUSIVE,
SHARE UPDATE EXCLUSIVE,
SHARE, SHARE ROW
EXCLUSIVE, EXCLUSIVE, and
ACCESS 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 of
ALTER TABLE.
EXCLUSIVE (ExclusiveLock)
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 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 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 DROP TABLE,
TRUNCATE, REINDEX,
CLUSTER, VACUUM FULL,
and REFRESH MATERIALIZED VIEW (without
CONCURRENTLY)
commands. Many forms of ALTER INDEX and ALTER TABLE also acquire
a lock at this level. This is also the default lock mode for
LOCK TABLE statements that do not specify
a mode explicitly.
Only an ACCESS EXCLUSIVE lock blocks a
SELECT (without FOR UPDATE/SHARE)
statement.
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. | |
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 |
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 PostgreSQL. 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 the
SELECT 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 attempt UPDATE,
DELETE,
SELECT FOR UPDATE,
SELECT FOR NO KEY UPDATE,
SELECT FOR SHARE or
SELECT 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 a REPEATABLE READ or
SERIALIZABLE 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 any DELETE on a row, and also by an
UPDATE that modifies the values of certain columns. Currently,
the set of columns considered for the UPDATE 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 block
SELECT FOR KEY SHARE commands that attempt to acquire
a lock on the same rows. This lock mode is also acquired by any
UPDATE that does not acquire a FOR 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 performing
UPDATE, DELETE,
SELECT FOR UPDATE or
SELECT FOR NO KEY UPDATE on these rows, but it does not
prevent them from performing SELECT FOR SHARE or
SELECT FOR KEY SHARE.
FOR KEY SHARE
Behaves similarly to FOR SHARE, except that the
lock is weaker: SELECT FOR UPDATE is blocked, but not
SELECT FOR NO KEY UPDATE. A key-shared lock blocks
other transactions from performing DELETE or
any UPDATE that changes the key values, but not
other UPDATE, and neither does it prevent
SELECT FOR NO KEY UPDATE, SELECT FOR SHARE,
or SELECT FOR KEY SHARE.
PostgreSQL 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