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><H1
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><A
NAME="VIEW-PG-LOCKS"
>41.33. <TT
CLASS="STRUCTNAME"
>pg_locks</TT
></A
></H1
><A
NAME="AEN55222"
></A
><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 12</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 may
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. A lockable object is either a relation (e.g., a
table) or a transaction ID.
</P
><P
> Note that this view includes only table-level
locks, not row-level ones. If a transaction is waiting for a
row-level lock, it will appear in the view as waiting for the
transaction ID of the current holder of that row lock.
</P
><DIV
CLASS="TABLE"
><A
NAME="AEN55230"
></A
><P
><B
>Table 41-33. <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"
>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 locked relation, or NULL if the lockable object
is a transaction ID
</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 locked relation exists, or
zero if the locked relation is a globally-shared table, or
NULL if the lockable object is a transaction ID
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>transaction</TT
></TD
><TD
><TT
CLASS="TYPE"
>xid</TT
></TD
><TD
> </TD
><TD
> ID of a transaction, or NULL if the lockable object is a relation
</TD
></TR
><TR
><TD
><TT
CLASS="STRUCTFIELD"
>pid</TT
></TD
><TD
><TT
CLASS="TYPE"
>integer</TT
></TD
><TD
> </TD
><TD
>process ID of a server process holding or awaiting this
lock</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 12.3.1</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
></TBODY
></TABLE
></DIV
><P
> <TT
CLASS="STRUCTFIELD"
>granted</TT
> is true in a row representing a lock
held by the indicated session. False indicates that this session is
currently waiting to acquire this lock, which implies that some other
session is holding a conflicting lock mode on the same lockable object.
The waiting session will sleep until the other lock is released (or a
deadlock situation is detected). A single session can be waiting to acquire
at most one lock at a time.
</P
><P
> Every transaction holds an exclusive lock on its transaction ID for its
entire duration. If 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. That will succeed only when the other transaction
terminates and releases its locks.
</P
><P
> When the <TT
CLASS="STRUCTNAME"
>pg_locks</TT
> view is accessed, the
internal lock manager data structures are momentarily locked, and
a copy is made for the view to display. This ensures that the
view produces a consistent set of results, while not blocking
normal lock manager operations longer than necessary. Nonetheless
there could be some impact on database performance if this view is
read often.
</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
> If you have enabled the statistics collector, the
<TT
CLASS="STRUCTFIELD"
>pid</TT
> column can be joined to the
<TT
CLASS="STRUCTFIELD"
>procpid</TT
> column of the
<TT
CLASS="STRUCTNAME"
>pg_stat_activity</TT
> view to get more
information on the session holding or waiting to hold the lock.
</P
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