in the same way as in <structname>pg_description</structname> or

<structname>pg_depend</structname>). Also, the right to extend a

relation is represented as a separate lockable object.

Also, <quote>advisory</> locks can be taken on numbers that have

user-defined meanings.

</para>

<table>

<entry>any OID column</entry>

<entry>

OID of the object within its system catalog, or null if the

object is not a general database object.

For advisory locks it is used to distinguish the two key

spaces (1 for an int8 key, 2 for two int4 keys).

object is not a general database object

</entry>

</row>

<row>

<entry><structfield>objsubid</structfield></entry>

<entry><type>smallint</type></entry>

<entry></entry>

<entry>

For a table column, this isthecolumn number (the

Column number targeted bythelock (the

<structfield>classid</> and <structfield>objid</> refer to the

table itself). For all other object types, this column is

zero. Null if the object is not a general database object

table itself),

or zero if the target is some other general database object,

or null if the target is not a general database object

</entry>

</row>

<row>

<entry></entry>

<entry>

Process ID of the server process holding or awaiting this

lock. Null if the lock is held by a prepared transaction.

lock, or null if the lock is held by a prepared transaction

</entry>

</row>

<row>

<para>

Advisory locks can be acquired on keys consisting of either a single

<type>bigint</type> value or two integer values. A <type>bigint</type> key is displayed with its

<type>bigint</type> value or two integer values.

A <type>bigint</type> key is displayed with its

high-order half in the <structfield>classid</> column, its low-order half

in the <structfield>objid</> column, and <structfield>objsubid</> equal

to 1. Integer keys are displayed with the first key in the

so the <structfield>database</> column is meaningful for an advisory lock.

</para>

<para>

The <structname>pg_locks</structname> view displays data from both the

regular lock manager and the predicate lock manager, which are

separate systems. When this view is accessed, the internal data

structures of each lock manager are momentarily locked, and copies are

made for the view to display. Each lock manager will therefore

produce 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. Each lock manager is only

locked for the minimum possible time so as to reduce the performance

impact of querying this view, but there could nevertheless be some

impact on database performance if it is frequently accessed.

</para>

<para>

<structname>pg_locks</structname> provides a global view of all locks

in the database cluster, not only those relevant to the current database.

but it continues to hold the locks it acquired while running.)

</para>

<para>

The <structname>pg_locks</structname> view displays data from both the

regular lock manager and the predicate lock manager, which are

separate systems. When this view is accessed, the internal data

structures of each lock manager are momentarily locked, and copies are

made for the view to display. Each lock manager will therefore

produce 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. Each lock manager is only

locked for the minimum possible time so as to reduce the performance

impact of querying this view, but there could nevertheless be some

impact on database performance if it is frequently accessed.

</para>

</sect1>

<sect1 id="view-pg-prepared-statements">