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Improve pg_autovacuum documentation to clarify that the enabled field cannot

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prevent anti-wraparound vacuuming, and to caution against setting unreasonably
small values of freeze_max_age.  Also put in a notice that this catalog is
likely to disappear entirely in some future release.  Per discussion of
bug #3898 from Steven Flatt.
This commit is contained in:
Tom Lane 2008-01-31 18:40:02 +00:00
parent 3c9ae7300d
commit 7c15b10a9a
1 changed files with 22 additions and 8 deletions
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30
doc/src/sgml/catalogs.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.160 2008/01/29 13:03:50 petere Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.161 2008/01/31 18:40:02 tgl Exp $ -->
<!--
Documentation of the system catalogs, directed toward PostgreSQL developers
-->
@ -1245,6 +1245,14 @@
about the autovacuum daemon, see <xref linkend="autovacuum">.
</para>
<note>
<para>
It is likely that <structname>pg_autovacuum</structname> will disappear
in a future release, with the information instead being kept in
<structname>pg_class</>.<structfield>reloptions</> entries.
</para>
</note>
<table>
<title><structname>pg_autovacuum</> Columns</title>
@ -1270,7 +1278,8 @@
<entry><structfield>enabled</structfield></entry>
<entry><type>bool</type></entry>
<entry></entry>
<entry>If false, this table is never autovacuumed</entry>
<entry>If false, this table will not be autovacuumed, except
to prevent transaction ID wraparound</entry>
</row>
<row>
@ -1352,7 +1361,9 @@
to prevent transaction ID wraparound if the table's
<structname>pg_class</>.<structfield>relfrozenxid</> field attains an age
of more than <structfield>freeze_max_age</> transactions, whether the table
has been changed or not. The system will launch autovacuum to perform
has been changed or not, even if
<structname>pg_autovacuum</>.<structfield>enabled</> is set to
<literal>false</> for it. The system will launch autovacuum to perform
such <command>VACUUM</>s even if autovacuum is otherwise disabled.
See <xref linkend="vacuum-for-wraparound"> for more about wraparound
prevention.
@ -1364,12 +1375,15 @@
be used for this particular value. Observe that the
<structfield>vac_cost_delay</> variable inherits its default value from the
<xref linkend="guc-autovacuum-vacuum-cost-delay"> configuration parameter,
or from <xref linkend="guc-vacuum-cost-delay"> if the former is set to a negative
value. The same applies to <structfield>vac_cost_limit</>.
or from <xref linkend="guc-vacuum-cost-delay"> if the former is set to a
negative value. The same applies to <structfield>vac_cost_limit</>.
Also, autovacuum will ignore attempts to set a per-table
<structfield>freeze_max_age</> larger than the system-wide setting (it can only be set
smaller), and the <structfield>freeze_min_age value</> will be limited to half the
system-wide <xref linkend="guc-autovacuum-freeze-max-age"> setting.
<structfield>freeze_max_age</> larger than the system-wide setting (it can
only be set smaller), and the <structfield>freeze_min_age</> value will be
limited to half the system-wide <xref
linkend="guc-autovacuum-freeze-max-age"> setting. Note that while you
can set <structfield>freeze_max_age</> very small, or even zero, this
is usually unwise since it will force frequent vacuuming.
</para>
</sect1>