querytree(to_tsquery('defined'));
 querytree
-----------
 'defin'

SELECT querytree(to_tsquery('!defined'));
 querytree
-----------
 T
</screen>
      </para>
     </listitem>
    </varlistentry>

   </variablelist>

   <sect3 id="textsearch-query-rewriting">
    <title>Query Rewriting</title>

    <indexterm zone="textsearch-query-rewriting">
     <primary>ts_rewrite</primary>
    </indexterm>

    <para>
     The <function>ts_rewrite</function> family of functions search a
     given <type>tsquery</type> for occurrences of a target
     subquery, and replace each occurrence with a
     substitute subquery.  In essence this operation is a
     <type>tsquery</type>-specific version of substring replacement.
     A target and substitute combination can be
     thought of as a <firstterm>query rewrite rule</firstterm>.  A collection
     of such rewrite rules can be a powerful search aid.
     For example, you can expand the search using synonyms
     (e.g., <literal>new york</literal>, <literal>big apple</literal>, <literal>nyc</literal>,
     <literal>gotham</literal>) or narrow the search to direct the user to some hot
     topic.  There is some overlap in functionality between this feature
     and thesaurus dictionaries (<xref linkend="textsearch-thesaurus"/>).
     However, you can modify a set of rewrite rules on-the-fly without
     reindexing, whereas updating a thesaurus requires reindexing to be
     effective.
    </para>

    <variablelist>

     <varlistentry>

      <term>
       <literal>ts_rewrite (<replaceable class="parameter">query</replaceable> <type>tsquery</type>, <replaceable class="parameter">target</replaceable> <type>tsquery</type>, <replaceable class="parameter">substitute</replaceable> <type>tsquery</type>) returns <type>tsquery</type></literal>
      </term>

      <listitem>
       <para>
        This form of <function>ts_rewrite</function> simply applies a single
        rewrite rule: <replaceable class="parameter">target</replaceable>
        is replaced by <replaceable class="parameter">substitute</replaceable>
        wherever it appears in <replaceable
        class="parameter">query</replaceable>.  For example:

<screen>
SELECT ts_rewrite('a &amp; b'::tsquery, 'a'::tsquery, 'c'::tsquery);
 ts_rewrite
------------
 'b' &amp; 'c'
</screen>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>

      <term>
       <literal>ts_rewrite (<replaceable class="parameter">query</replaceable> <type>tsquery</type>, <replaceable class="parameter">select</replaceable> <type>text</type>) returns <type>tsquery</type></literal>
      </term>

      <listitem>
       <para>
        This form of <function>ts_rewrite</function> accepts a starting
        <replaceable>query</replaceable> and an SQL <replaceable>select</replaceable> command, which
        is given as a text string.  The <replaceable>select</replaceable> must yield two
        columns of <type>tsquery</type> type.  For each row of the
        <replaceable>select</replaceable> result, occurrences of the first column value
        (the target) are replaced by the second column value (the substitute)
        within the current <replaceable>query</replaceable> value.  For example:

<screen>
CREATE TABLE aliases (t tsquery PRIMARY KEY, s tsquery);
INSERT INTO aliases VALUES('a', 'c');

SELECT ts_rewrite('a &amp; b'::tsquery, 'SELECT t,s FROM aliases');
 ts_rewrite
------------
 'b' &amp; 'c'
</screen>
       </para>

       <para>
        Note that when multiple rewrite rules are applied in this way,
        the order of application can be important; so in practice you will
        want the source query to <literal>ORDER BY</literal> some ordering key.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>

    <para>
     Let's consider a real-life astronomical example. We'll expand query
     <literal>supernovae</literal> using table-driven rewriting rules:

<screen>
CREATE TABLE aliases (t tsquery primary key,