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Documentation improvements for pg_locks with respect to SSI.

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Explain that querying pg_locks does not simultaneously lock both the
normal lock manager and the predicate lock manager.

Per discussion with Kevin Grittner.
This commit is contained in:
Robert Haas 2011-06-24 16:06:57 -04:00
parent 194e6ee74b
commit eb03cff820
1 changed files with 12 additions and 7 deletions
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19
doc/src/sgml/catalogs.sgml
View File

@ -7085,13 +7085,18 @@
</para>
<para>
When the <structname>pg_locks</structname> 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
frequently accessed.
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>