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
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>
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