Index: doc/src/sgml/ref/create_index.sgml
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RCS file: /cvsroot/pgsql-server/doc/src/sgml/ref/create_index.sgml,v
retrieving revision 1.44
diff -c -c -r1.44 create_index.sgml
*** doc/src/sgml/ref/create_index.sgml	29 Nov 2003 19:51:38 -0000	1.44
--- doc/src/sgml/ref/create_index.sgml	20 Apr 2004 00:54:29 -0000
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*** 66,72 ****
      When the <literal>WHERE</literal> clause is present, a
      <firstterm>partial index</firstterm> is created.
      A partial index is an index that contains entries for only a portion of
!     a table, usually a portion that is somehow more interesting than the
      rest of the table. For example, if you have a table that contains both
      billed and unbilled orders where the unbilled orders take up a small
      fraction of the total table and yet that is an often used section, you
--- 66,72 ----
      When the <literal>WHERE</literal> clause is present, a
      <firstterm>partial index</firstterm> is created.
      A partial index is an index that contains entries for only a portion of
!     a table, usually a portion that is more useful for indexing than the
      rest of the table. For example, if you have a table that contains both
      billed and unbilled orders where the unbilled orders take up a small
      fraction of the total table and yet that is an often used section, you
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*** 77,85 ****
    </para>
  
    <para>
      The expression used in the <literal>WHERE</literal> clause may refer
!     only to columns of the underlying table (but it can use all columns,
!     not only the one(s) being indexed).  Presently, subqueries and
      aggregate expressions are also forbidden in <literal>WHERE</literal>.
      The same restrictions apply to index fields that are expressions.
    </para>
--- 77,93 ----
    </para>
  
    <para>
+    <literal>NULL</> values are not indexed by default.  The best way
+    to index <literal>NULL</> values is to create a partial index using
+    an <literal>IS NULL</> comparison.  <literal>IS NULL</> is more
+    of a function call than a value comparison, and this is why a partial
+    index works.
+   </para>
+ 
+   <para>
      The expression used in the <literal>WHERE</literal> clause may refer
!     only to columns of the underlying table, but it can use all columns,
!     not just the ones being indexed.  Presently, subqueries and
      aggregate expressions are also forbidden in <literal>WHERE</literal>.
      The same restrictions apply to index fields that are expressions.
    </para>