example, normally <literal>!x</literal> matches
    only documents that do not contain <literal>x</literal> anywhere.
    But <literal>!x &lt;-&gt; y</literal> matches <literal>y</literal> if it is not
    immediately after an <literal>x</literal>; an occurrence of <literal>x</literal>
    elsewhere in the document does not prevent a match.  Another example is
    that <literal>x &amp; y</literal> normally only requires that <literal>x</literal>
    and <literal>y</literal> both appear somewhere in the document, but
    <literal>(x &amp; y) &lt;-&gt; z</literal> requires <literal>x</literal>
    and <literal>y</literal> to match at the same place, immediately before
    a <literal>z</literal>.  Thus this query behaves differently from
    <literal>x &lt;-&gt; z &amp; y &lt;-&gt; z</literal>, which will match a
    document containing two separate sequences <literal>x z</literal> and
    <literal>y z</literal>.  (This specific query is useless as written,
    since <literal>x</literal> and <literal>y</literal> could not match at the same place;
    but with more complex situations such as prefix-match patterns, a query
    of this form could be useful.)
   </para>
  </sect2>

  <sect2 id="textsearch-intro-configurations">
   <title>Configurations</title>

   <para>
    The above are all simple text search examples.  As mentioned before, full
    text search functionality includes the ability to do many more things:
    skip indexing certain words (stop words), process synonyms, and use
    sophisticated parsing, e.g., parse based on more than just white space.
    This functionality is controlled by <firstterm>text search
    configurations</firstterm>.  <productname>PostgreSQL</productname> comes with predefined
    configurations for many languages, and you can easily create your own
    configurations.  (<application>psql</application>'s <command>\dF</command> command
    shows all available configurations.)
   </para>

   <para>
    During installation an appropriate configuration is selected and
    <xref linkend="guc-default-text-search-config"/> is set accordingly
    in <filename>postgresql.conf</filename>.  If you are using the same text search
    configuration for the entire cluster you can use the value in
    <filename>postgresql.conf</filename>.  To use different configurations
    throughout the cluster but the same configuration within any one database,
    use <command>ALTER DATABASE ... SET</command>.  Otherwise, you can set
    <varname>default_text_search_config</varname> in each session.
   </para>

   <para>
    Each text search function that depends on a configuration has an optional
    <type>regconfig</type> argument, so that the configuration to use can be
    specified explicitly.  <varname>default_text_search_config</varname>
    is used only when this argument is omitted.
   </para>

   <para>
    To make it easier to build custom text search configurations, a
    configuration is built up from simpler database objects.
    <productname>PostgreSQL</productname>'s text search facility provides
    four types of configuration-related database objects:
   </para>

  <itemizedlist  spacing="compact" mark="bullet">
   <listitem>
    <para>
     <firstterm>Text search parsers</firstterm> break documents into tokens
     and classify each token (for example, as words or numbers).
    </para>
   </listitem>

   <listitem>
    <para>
     <firstterm>Text search dictionaries</firstterm> convert tokens to normalized
     form and reject stop words.
    </para>
   </listitem>

   <listitem>
    <para>
     <firstterm>Text search templates</firstterm> provide the functions underlying
     dictionaries.  (A dictionary simply specifies a template and a set
     of parameters for the template.)
    </para>
   </listitem>

   <listitem>
    <para>
     <firstterm>Text search configurations</firstterm> select a parser and a set
     of dictionaries to use to normalize the tokens produced by the parser.
    </para>