<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, s tsquery);
INSERT INTO aliases VALUES(to_tsquery('supernovae'), to_tsquery('supernovae|sn'));

SELECT ts_rewrite(to_tsquery('supernovae &amp; crab'), 'SELECT * FROM aliases');
           ts_rewrite
---------------------------------
 'crab' &amp; ( 'supernova' | 'sn' )
</screen>

     We can change the rewriting rules just by updating the table:

<screen>
UPDATE aliases
SET s = to_tsquery('supernovae|sn &amp; !nebulae')
WHERE t = to_tsquery('supernovae');

SELECT ts_rewrite(to_tsquery('supernovae &amp; crab'), 'SELECT * FROM aliases');
                 ts_rewrite
---------------------------------------------
 'crab' &amp; ( 'supernova' | 'sn' &amp; !'nebula' )
</screen>
    </para>

    <para>
     Rewriting can be slow when there are many rewriting rules, since it
     checks every rule for a possible match. To filter out obvious non-candidate
     rules we can use the containment operators for the <type>tsquery</type>
     type. In the example below, we select only those rules which might match
     the original query:

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

   </sect3>

  </sect2>

  <sect2 id="textsearch-update-triggers">
   <title>Triggers for Automatic Updates</title>

   <indexterm>
    <primary>trigger</primary>
    <secondary>for updating a derived tsvector column</secondary>
   </indexterm>

   <note>
    <para>
     The method described in this section has been obsoleted by the use of
     stored generated columns, as described in <xref
     linkend="textsearch-tables-index"/>.
    </para>
   </note>

   <para>
    When using a separate column to store the <type>tsvector</type> representation
    of your documents, it is necessary to create a trigger to update the
    <type>tsvector</type> column when the document content columns change.
    Two built-in trigger functions are available for this, or you can write
    your own.
   </para>

<synopsis>
tsvector_update_trigger(<replaceable class="parameter">tsvector_column_name</replaceable>,&zwsp; <replaceable class="parameter">config_name</replaceable>, <replaceable class="parameter">text_column_name</replaceable> <optional>, ... </optional>)
tsvector_update_trigger_column(<replaceable class="parameter">tsvector_column_name</replaceable>,&zwsp; <replaceable class="parameter">config_column_name</replaceable>, <replaceable class="parameter">text_column_name</replaceable> <optional>, ... </optional>)
</synopsis>

   <para>
    These trigger functions automatically compute a <type>tsvector</type>
    column from one or more textual columns, under the control of
    parameters specified in the <command>CREATE TRIGGER</command> command.
    An example of their use is:

<screen>
CREATE TABLE messages (
    title       text,
    body        text,
    tsv         tsvector
);

CREATE TRIGGER tsvectorupdate BEFORE INSERT OR UPDATE
ON messages