Doc updates for index-only scans.

Document that routine vacuuming is now also important for the purpose
of index-only scans; and mention in the section that describes the
visibility map that it is used to implement index-only scans.

Marti Raudsepp, with some changes by me.

doc/src/sgml/maintenance.sgml

@@ -101,6 +101,11 @@
       <productname>PostgreSQL</productname> query planner.</simpara>
      </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>
@@ -329,6 +334,33 @@
    </tip>
   </sect2>
 
+  <sect2 id="vacuum-for-visibility-map">
+   <title>Updating The Visibility Map</title>
+
+   <para>
+    Vacuum maintains a <link linkend="storage-vm">visibility map</> for each
+    table to keep track of which pages contain only tuples that are known to be
+    visible to all active transactions (and all future transactions, until the
+    page is again modified).  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, it allows <productname>PostgreSQL</productname> to answer some
+    queries using only the index, without reference to the underlying table.
+    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 <firstterm>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.
+    The visibility map is vastly smaller than the heap, so it can easily be
+    cached even when the heap is very large.
+   </para>
+  </sect2>
+
   <sect2 id="vacuum-for-wraparound">
    <title>Preventing Transaction ID Wraparound Failures</title>