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><DIV
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><H1
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><A
NAME="VIEW-PG-LOCKS"
>45.58. <TT
CLASS="STRUCTNAME"
>pg_locks</TT
></A
></H1
><P
> The view <TT
CLASS="STRUCTNAME"
>pg_locks</TT
> provides access to
information about the locks held by open transactions within the
database server. See <A
HREF="mvcc.html"
>Chapter 13</A
> for more discussion
of locking.
</P
><P
> <TT
CLASS="STRUCTNAME"
>pg_locks</TT
> contains one row per active lockable
object, requested lock mode, and relevant transaction. Thus, the same
lockable object might
appear many times, if multiple transactions are holding or waiting
for locks on it. However, an object that currently has no locks on it
will not appear at all.
</P
><P
> 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 <TT
CLASS="STRUCTNAME"
>pg_description</TT
> or
<TT
CLASS="STRUCTNAME"
>pg_depend</TT
>). Also, the right to extend a
relation is represented as a separate lockable object.
Also, <SPAN
CLASS="QUOTE"
>"advisory"</SPAN
> locks can be taken on numbers that have
user-defined meanings.
</P
><DIV
CLASS="TABLE"
><A
NAME="AEN94330"
></A
><P
><B
>Table 45-59. <TT
CLASS="STRUCTNAME"
>pg_locks</TT
> Columns</B
></P
><TABLE
BORDER="1"
CLASS="CALSTABLE"
><COL><COL><COL><COL><THEAD
><TR
><TH
>Name</TH
><TH
>Type</TH
><TH
>References</TH
><TH
>Description</TH
></TR
></THEAD
><TBODY
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>locktype</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
> </TD
><TD
> Type of the lockable object:
<TT
CLASS="LITERAL"
>relation</TT
>,
<TT
CLASS="LITERAL"
>extend</TT
>,
<TT
CLASS="LITERAL"
>page</TT
>,
<TT
CLASS="LITERAL"
>tuple</TT
>,
<TT
CLASS="LITERAL"
>transactionid</TT
>,
<TT
CLASS="LITERAL"
>virtualxid</TT
>,
<TT
CLASS="LITERAL"
>object</TT
>,
<TT
CLASS="LITERAL"
>userlock</TT
>, or
<TT
CLASS="LITERAL"
>advisory</TT
>
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>database</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-database.html"
><TT
CLASS="STRUCTNAME"
>pg_database</TT
></A
>.oid</TT
></TD
><TD
> 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
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>relation</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-class.html"
><TT
CLASS="STRUCTNAME"
>pg_class</TT
></A
>.oid</TT
></TD
><TD
> OID of the relation targeted by the lock, or null if the target is not
a relation or part of a relation
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>page</TT
></TD
><TD
><TT
CLASS="TYPE"
>integer</TT
></TD
><TD
> </TD
><TD
> Page number targeted by the lock within the relation,
or null if the target is not a relation page or tuple
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>tuple</TT
></TD
><TD
><TT
CLASS="TYPE"
>smallint</TT
></TD
><TD
> </TD
><TD
> Tuple number targeted by the lock within the page,
or null if the target is not a tuple
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>virtualxid</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
> </TD
><TD
> Virtual ID of the transaction targeted by the lock,
or null if the target is not a virtual transaction ID
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>transactionid</TT
></TD
><TD
><TT
CLASS="TYPE"
>xid</TT
></TD
><TD
> </TD
><TD
> ID of the transaction targeted by the lock,
or null if the target is not a transaction ID
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>classid</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
><TT
CLASS="LITERAL"
><A
HREF="catalog-pg-class.html"
><TT
CLASS="STRUCTNAME"
>pg_class</TT
></A
>.oid</TT
></TD
><TD
> OID of the system catalog containing the lock target, or null if the
target is not a general database object
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>objid</TT
></TD
><TD
><TT
CLASS="TYPE"
>oid</TT
></TD
><TD
>any OID column</TD
><TD
> OID of the lock target within its system catalog, or null if the
target is not a general database object
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>objsubid</TT
></TD
><TD
><TT
CLASS="TYPE"
>smallint</TT
></TD
><TD
> </TD
><TD
> Column number targeted by the lock (the
<TT
CLASS="STRUCTFIELD"
>classid</TT
> and <TT
CLASS="STRUCTFIELD"
>objid</TT
> refer to the
table itself),
or zero if the target is some other general database object,
or null if the target is not a general database object
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>virtualtransaction</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
> </TD
><TD
> Virtual ID of the transaction that is holding or awaiting this lock
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>pid</TT
></TD
><TD
><TT
CLASS="TYPE"
>integer</TT
></TD
><TD
> </TD
><TD
> Process ID of the server process holding or awaiting this
lock, or null if the lock is held by a prepared transaction
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>mode</TT
></TD
><TD
><TT
CLASS="TYPE"
>text</TT
></TD
><TD
> </TD
><TD
>Name of the lock mode held or desired by this process (see <A
HREF="explicit-locking.html#LOCKING-TABLES"
>Section 13.3.1</A
> and <A
HREF="transaction-iso.html#XACT-SERIALIZABLE"
>Section 13.2.3</A
>)</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>granted</TT
></TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
> </TD
><TD
>True if lock is held, false if lock is awaited</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>fastpath</TT
></TD
><TD
><TT
CLASS="TYPE"
>boolean</TT
></TD
><TD
> </TD
><TD
>True if lock was taken via fast path, false if taken via main
lock table</TD
></TR
></TBODY
></TABLE
></DIV
><P
> <TT
CLASS="STRUCTFIELD"
>granted</TT
> is true in a row representing a lock
held by the indicated transaction. False indicates that this transaction is
currently waiting to acquire this lock, which implies that some other
transaction is holding a conflicting lock mode on the same lockable object.
The waiting transaction will sleep until the other lock is released (or a
deadlock situation is detected). A single transaction can be waiting to
acquire at most one lock at a time.
</P
><P
> Every transaction holds an exclusive lock on its virtual transaction ID for
its entire duration. 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 its permanent transaction ID
until it ends. When one transaction finds it necessary to wait specifically
for another transaction, it does so by attempting to acquire share lock on
the other transaction ID (either virtual or permanent ID depending on the
situation). That will succeed only when the other transaction
terminates and releases its locks.
</P
><P
> 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 transaction 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.
</P
><P
> Advisory locks can be acquired on keys consisting of either a single
<TT
CLASS="TYPE"
>bigint</TT
> value or two integer values.
A <TT
CLASS="TYPE"
>bigint</TT
> key is displayed with its
high-order half in the <TT
CLASS="STRUCTFIELD"
>classid</TT
> column, its low-order half
in the <TT
CLASS="STRUCTFIELD"
>objid</TT
> column, and <TT
CLASS="STRUCTFIELD"
>objsubid</TT
> equal
to 1. Integer keys are displayed with the first key in the
<TT
CLASS="STRUCTFIELD"
>classid</TT
> column, the second key in the <TT
CLASS="STRUCTFIELD"
>objid</TT
>
column, and <TT
CLASS="STRUCTFIELD"
>objsubid</TT
> equal to 2. The actual meaning of
the keys is up to the user. Advisory locks are local to each database,
so the <TT
CLASS="STRUCTFIELD"
>database</TT
> column is meaningful for an advisory lock.
</P
><P
> <TT
CLASS="STRUCTNAME"
>pg_locks</TT
> provides a global view of all locks
in the database cluster, not only those relevant to the current database.
Although its <TT
CLASS="STRUCTFIELD"
>relation</TT
> column can be joined
against <TT
CLASS="STRUCTNAME"
>pg_class</TT
>.<TT
CLASS="STRUCTFIELD"
>oid</TT
> to identify locked
relations, this will only work correctly for relations in the current
database (those for which the <TT
CLASS="STRUCTFIELD"
>database</TT
> column
is either the current database's OID or zero).
</P
><P
> The <TT
CLASS="STRUCTFIELD"
>pid</TT
> column can be joined to the
<TT
CLASS="STRUCTFIELD"
>pid</TT
> column of the <A
HREF="monitoring-stats.html#PG-STAT-ACTIVITY-VIEW"
><TT
CLASS="STRUCTNAME"
>pg_stat_activity</TT
></A
>
view to get more
information on the session holding or waiting to hold each lock,
for example
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM pg_locks pl LEFT JOIN pg_stat_activity psa
ON pl.pid = psa.pid;</PRE
><P>
Also, if you are using prepared transactions, the
<TT
CLASS="STRUCTFIELD"
>virtualtransaction</TT
> column can be joined to the
<TT
CLASS="STRUCTFIELD"
>transaction</TT
> column of the <A
HREF="view-pg-prepared-xacts.html"
><TT
CLASS="STRUCTNAME"
>pg_prepared_xacts</TT
></A
>
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:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
ON pl.virtualtransaction = '-1/' || ppx.transaction;</PRE
><P>
</P
><P
> The <TT
CLASS="STRUCTNAME"
>pg_locks</TT
> 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 <I
CLASS="FIRSTTERM"
>fast-path</I
> locks.
This data is not guaranteed to be entirely consistent.
When the view is queried,
data on fast-path locks (with <TT
CLASS="STRUCTFIELD"
>fastpath</TT
> = <TT
CLASS="LITERAL"
>true</TT
>)
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.
</P
><P
> 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.
</P
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