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* Update documentation wrt visibility map/index-only scans
@ 2012-03-03 00:16 Marti Raudsepp <marti@juffo.org>
2012-03-16 10:08 ` Re: Update documentation wrt visibility map/index-only scans Marti Raudsepp <marti@juffo.org>
2012-03-21 18:53 ` Re: Update documentation wrt visibility map/index-only scans Robert Haas <robertmhaas@gmail.com>
0 siblings, 2 replies; 4+ messages in thread
From: Marti Raudsepp @ 2012-03-03 00:16 UTC (permalink / raw)
To: pgsql-docs
Hi list,
The documentation needs some updates now that we have index-only
scans. Patch attached. Perhaps the wording can be improved by someone
whose native language is English. But something like this should be
added.
(Not tested, as I couldn't figure out how to build docs on my system)
Regards,
Marti
Attachments:
index-only-docs.patch (text/x-patch, 3.2K) download
diff --git a/doc/src/sgml/maintenance.sgml b/doc/src/sgml/maintenance.sgml
new file mode 100644
index 03cc6c9..e042293
*** a/doc/src/sgml/maintenance.sgml
--- b/doc/src/sgml/maintenance.sgml
***************
*** 102,107 ****
--- 102,112 ----
</listitem>
<listitem>
+ <simpara>To update the visibility map, which speeds up index-only
+ scans.</simpara>
+ </listitem>
+
+ <listitem>
<simpara>To protect against loss of very old data due to
<firstterm>transaction ID wraparound</>.</simpara>
</listitem>
***************
*** 329,334 ****
--- 334,361 ----
</tip>
</sect2>
+ <sect2 id="vacuum-for-visibility-map">
+ <title>Updating Visibility Map</title>
+
+ <para>
+ Vacuum maintains the table <xref linkend="storage-vm">, which keeps track
+ of pages that only contain tuples that are visible to all transactions.
+ This has two purposes. First, vacuum itself can skip such pages on the
+ next run, since there is nothing to clean up.
+ </para>
+
+ <para>
+ Second, index-only scans also take advantage of the visibility map. Since
+ <productname>PostgreSQL</productname> indexes don't contain tuple
+ visibility information, a normal index scan fetches the heap tuple for each
+ matching index entry, to check whether it should be seen by the current
+ transaction. An index-only scan, on the other hand, checks the
+ visibility map first. If it's known that all tuples on the page are
+ visible, the heap fetch can be skipped. This is most noticeable on
+ large data sets where the visibility map can prevent disk accesses.
+ </para>
+ </sect2>
+
<sect2 id="vacuum-for-wraparound">
<title>Preventing Transaction ID Wraparound Failures</title>
diff --git a/doc/src/sgml/storage.sgml b/doc/src/sgml/storage.sgml
new file mode 100644
index cb2f60e..a1c300d
*** a/doc/src/sgml/storage.sgml
--- b/doc/src/sgml/storage.sgml
*************** Note that indexes do not have VMs.
*** 494,504 ****
<para>
The visibility map simply stores one bit per heap page. A set bit means
that all tuples on the page are known to be visible to all transactions.
! This means that the page does not contain any tuples that need to be vacuumed;
! in future it might also be used to avoid visiting the page for visibility
! checks. The map is conservative in the sense that we
! make sure that whenever a bit is set, we know the condition is true, but if
! a bit is not set, it might or might not be true.
</para>
</sect1>
--- 494,509 ----
<para>
The visibility map simply stores one bit per heap page. A set bit means
that all tuples on the page are known to be visible to all transactions.
! This information is used by vacuum to skip pages that do not contain any
! tuples that need to be vacuumed and by <fistterm>index-only scans</> to skip
! row visibility checks to the table heap.
! </para>
!
! <para>
! The map is conservative in the sense that we make sure that whenever a bit is
! set, we know the condition is true, but if a bit is not set, it might or
! might not be true. Visibility map bits are only set by vacuum, but are
! cleared by any data-modifying operations on a page.
</para>
</sect1>
* Re: Update documentation wrt visibility map/index-only scans
2012-03-03 00:16 Update documentation wrt visibility map/index-only scans Marti Raudsepp <marti@juffo.org>
@ 2012-03-16 10:08 ` Marti Raudsepp <marti@juffo.org>
2012-03-16 14:01 ` Re: Update documentation wrt visibility map/index-only scans Alvaro Herrera <alvherre@commandprompt.com>
1 sibling, 1 reply; 4+ messages in thread
From: Marti Raudsepp @ 2012-03-16 10:08 UTC (permalink / raw)
To: pgsql-docs
On Sat, Mar 3, 2012 at 02:16, Marti Raudsepp <marti@juffo.org> wrote:
> The documentation needs some updates now that we have index-only
> scans.
Bump. No response for 2 weeks. Anyone?
Regards,
Marti
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Update documentation wrt visibility map/index-only scans
2012-03-03 00:16 Update documentation wrt visibility map/index-only scans Marti Raudsepp <marti@juffo.org>
2012-03-16 10:08 ` Re: Update documentation wrt visibility map/index-only scans Marti Raudsepp <marti@juffo.org>
@ 2012-03-16 14:01 ` Alvaro Herrera <alvherre@commandprompt.com>
0 siblings, 0 replies; 4+ messages in thread
From: Alvaro Herrera @ 2012-03-16 14:01 UTC (permalink / raw)
To: Marti Raudsepp <marti@juffo.org>; +Cc: pgsql-docs
Excerpts from Marti Raudsepp's message of vie mar 16 07:08:18 -0300 2012:
> On Sat, Mar 3, 2012 at 02:16, Marti Raudsepp <marti@juffo.org> wrote:
> > The documentation needs some updates now that we have index-only
> > scans.
>
> Bump. No response for 2 weeks. Anyone?
Add it here:
http://wiki.postgresql.org/wiki/Open_Items
--
Álvaro Herrera <alvherre@commandprompt.com>
The PostgreSQL Company - Command Prompt, Inc.
PostgreSQL Replication, Consulting, Custom Development, 24x7 support
^ permalink raw reply [nested|flat] 4+ messages in thread
* Re: Update documentation wrt visibility map/index-only scans
2012-03-03 00:16 Update documentation wrt visibility map/index-only scans Marti Raudsepp <marti@juffo.org>
@ 2012-03-21 18:53 ` Robert Haas <robertmhaas@gmail.com>
1 sibling, 0 replies; 4+ messages in thread
From: Robert Haas @ 2012-03-21 18:53 UTC (permalink / raw)
To: Marti Raudsepp <marti@juffo.org>; +Cc: pgsql-docs
On Fri, Mar 2, 2012 at 7:16 PM, Marti Raudsepp <marti@juffo.org> wrote:
> The documentation needs some updates now that we have index-only
> scans. Patch attached. Perhaps the wording can be improved by someone
> whose native language is English. But something like this should be
> added.
>
> (Not tested, as I couldn't figure out how to build docs on my system)
Committed after some editing and markup repair.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
^ permalink raw reply [nested|flat] 4+ messages in thread
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2012-03-03 00:16 Update documentation wrt visibility map/index-only scans Marti Raudsepp <marti@juffo.org>
2012-03-16 10:08 ` Marti Raudsepp <marti@juffo.org>
2012-03-16 14:01 ` Alvaro Herrera <alvherre@commandprompt.com>
2012-03-21 18:53 ` Robert Haas <robertmhaas@gmail.com>
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