pg_locks
The view pg_locks provides access to
information about the locks held by active processes within the
database server. See Chapter 13 for more discussion
of locking.
pg_locks contains one row per active lockable
object, requested lock mode, and relevant process. Thus, the same
lockable object might
appear many times, if multiple processes are holding or waiting
for locks on it. However, an object that currently has no locks on it
will not appear at all.
There are several distinct types of lockable objects:
whole relations (e.g., tables), individual pages of relations,
individual tuples of relations,
transaction IDs (both virtual and permanent IDs),
and general database objects (identified by class OID and object OID,
in the same way as in pg_description or
pg_depend). Also, the right to extend a
relation is represented as a separate lockable object, as is the right to
update pg_database.datfrozenxid.
Also, “advisory” locks can be taken on numbers that have
user-defined meanings.
Table 51.75. pg_locks Columns
Column Type Description |
|---|
Type of the lockable object:
|
OID of the database in which the lock target exists, or zero if the target is a shared object, or null if the target is a transaction ID |
OID of the relation targeted by the lock, or null if the target is not a relation or part of a relation |
Page number targeted by the lock within the relation, or null if the target is not a relation page or tuple |
Tuple number targeted by the lock within the page, or null if the target is not a tuple |
Virtual ID of the transaction targeted by the lock, or null if the target is not a virtual transaction ID |
ID of the transaction targeted by the lock, or null if the target is not a transaction ID |
OID of the system catalog containing the lock target, or null if the target is not a general database object |
OID of the lock target within its system catalog, or null if the target is not a general database object |
Column number targeted by the lock (the
|
Virtual ID of the transaction that is holding or awaiting this lock |
Process ID of the server process holding or awaiting this lock, or null if the lock is held by a prepared transaction |
Name of the lock mode held or desired by this process (see Section 13.3.1 and Section 13.2.3) |
True if lock is held, false if lock is awaited |
True if lock was taken via fast path, false if taken via main lock table |
granted is true in a row representing a lock
held by the indicated process. False indicates that this process is
currently waiting to acquire this lock, which implies that at least one
other process is holding or waiting for a conflicting lock mode on the same
lockable object. The waiting process will sleep until the other lock is
released (or a deadlock situation is detected). A single process can be
waiting to acquire at most one lock at a time.
Throughout running a transaction, a server process holds an exclusive lock on the transaction's virtual transaction ID. If a permanent ID is assigned to the transaction (which normally happens only if the transaction changes the state of the database), it also holds an exclusive lock on the transaction's permanent transaction ID until it ends. When a process finds it necessary to wait specifically for another transaction to end, it does so by attempting to acquire share lock on the other transaction's ID (either virtual or permanent ID depending on the situation). That will succeed only when the other transaction terminates and releases its locks.
Although tuples are a lockable type of object, information about row-level locks is stored on disk, not in memory, and therefore row-level locks normally do not appear in this view. If a process is waiting for a row-level lock, it will usually appear in the view as waiting for the permanent transaction ID of the current holder of that row lock.
Advisory locks can be acquired on keys consisting of either a single
bigint value or two integer values.
A bigint key is displayed with its
high-order half in the classid column, its low-order half
in the objid column, and objsubid equal
to 1. The original bigint value can be reassembled with the
expression (classid::bigint << 32) |
objid::bigint. Integer keys are displayed with the
first key in the
classid column, the second key in the objid
column, and objsubid equal to 2. The actual meaning of
the keys is up to the user. Advisory locks are local to each database,
so the database column is meaningful for an advisory lock.
pg_locks provides a global view of all locks
in the database cluster, not only those relevant to the current database.
Although its relation column can be joined
against pg_class.oid to identify locked
relations, this will only work correctly for relations in the current
database (those for which the database column
is either the current database's OID or zero).
The pid column can be joined to the
pid column of the
pg_stat_activity
view to get more
information on the session holding or awaiting each lock,
for example
SELECT * FROM pg_locks pl LEFT JOIN pg_stat_activity psa
ON pl.pid = psa.pid;
Also, if you are using prepared transactions, the
virtualtransaction column can be joined to the
transaction column of the pg_prepared_xacts
view to get more information on prepared transactions that hold locks.
(A prepared transaction can never be waiting for a lock,
but it continues to hold the locks it acquired while running.)
For example:
SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
ON pl.virtualtransaction = '-1/' || ppx.transaction;
While it is possible to obtain information about which processes block
which other processes by joining pg_locks against
itself, this is very difficult to get right in detail. Such a query would
have to encode knowledge about which lock modes conflict with which
others. Worse, the pg_locks view does not expose
information about which processes are ahead of which others in lock wait
queues, nor information about which processes are parallel workers running
on behalf of which other client sessions. It is better to use
the pg_blocking_pids() function
(see Table 9.63) to identify which
process(es) a waiting process is blocked behind.
The pg_locks view displays data from both the
regular lock manager and the predicate lock manager, which are
separate systems; in addition, the regular lock manager subdivides its
locks into regular and fast-path locks.
This data is not guaranteed to be entirely consistent.
When the view is queried,
data on fast-path locks (with fastpath = true)
is gathered from each backend one at a time, without freezing the state of
the entire lock manager, so it is possible for locks to be taken or
released while information is gathered. Note, however, that these locks are
known not to conflict with any other lock currently in place. After
all backends have been queried for fast-path locks, the remainder of the
regular lock manager is locked as a unit, and a consistent snapshot of all
remaining locks is collected as an atomic action. After unlocking the
regular lock manager, the predicate lock manager is similarly locked and all
predicate locks are collected as an atomic action. Thus, with the exception
of fast-path locks, each lock manager will deliver a consistent set of
results, but as we do not lock both lock managers simultaneously, it is
possible for locks to be taken or released after we interrogate the regular
lock manager and before we interrogate the predicate lock manager.
Locking the regular and/or predicate lock manager could have some impact on database performance if this view is very frequently accessed. The locks are held only for the minimum amount of time necessary to obtain data from the lock managers, but this does not completely eliminate the possibility of a performance impact.