Clarify use of btree indexes for ILIKE and ~*.

doc/src/sgml/indices.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/indices.sgml,v 1.55 2005/11/07 17:36:44 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/indices.sgml,v 1.56 2006/01/18 21:29:45 momjian Exp $ -->
 
 <chapter id="indexes">
  <title id="indexes-title">Indexes</title>
@@ -141,17 +141,21 @@ CREATE INDEX test1_id_index ON test1 (id);
 
   <para>
    The optimizer can also use a B-tree index for queries involving the
-   pattern matching operators <literal>LIKE</>,
-   <literal>ILIKE</literal>, <literal>~</literal>, and
-   <literal>~*</literal>, <emphasis>if</emphasis> the pattern is a constant
-   and is anchored to the beginning of the string &mdash; for example,
-   <literal>col LIKE 'foo%'</literal> or <literal>col ~ '^foo'</literal>,
-   but not <literal>col LIKE '%bar'</literal>.  However, if your server does
-   not use the C locale you will need to create the index with a
-   special operator class to support indexing of pattern-matching queries.
-   See <xref linkend="indexes-opclass"> below.
+   pattern matching operators <literal>LIKE</> and <literal>~</literal>
+   <emphasis>if</emphasis> the pattern is a constant and is anchored to
+   the beginning of the string &mdash; for example, <literal>col LIKE
+   'foo%'</literal> or <literal>col ~ '^foo'</literal>, but not
+   <literal>col LIKE '%bar'</literal>. However, if your server does not
+   use the C locale you will need to create the index with a special
+   operator class to support indexing of pattern-matching queries. See
+   <xref linkend="indexes-opclass"> below. It is also possible to use
+   B-tree indexes for <literal>ILIKE</literal> and
+   <literal>~*</literal>, but only if the pattern starts with
+   non-alphabetic characters, i.e. characters that are not affected by
+   upper/lower case conversion.
   </para>
 
+
   <para>
    <indexterm>
     <primary>index</primary>