like the title or an initial abstract, so they can be
    treated with more or less importance than words in the document body.
   </para>

   <para>
    Since a longer document has a greater chance of containing a query term
    it is reasonable to take into account document size, e.g., a hundred-word
    document with five instances of a search word is probably more relevant
    than a thousand-word document with five instances.  Both ranking functions
    take an integer <replaceable>normalization</replaceable> option that
    specifies whether and how a document's length should impact its rank.
    The integer option controls several behaviors, so it is a bit mask:
    you can specify one or more behaviors using
    <literal>|</literal> (for example, <literal>2|4</literal>).

    <itemizedlist  spacing="compact" mark="bullet">
     <listitem>
      <para>
       0 (the default) ignores the document length
      </para>
     </listitem>
     <listitem>
      <para>
       1 divides the rank by 1 + the logarithm of the document length
      </para>
     </listitem>
     <listitem>
      <para>
       2 divides the rank by the document length
      </para>
     </listitem>
     <listitem>
      <para>
       4 divides the rank by the mean harmonic distance between extents
       (this is implemented only by <function>ts_rank_cd</function>)
      </para>
     </listitem>
     <listitem>
      <para>
       8 divides the rank by the number of unique words in document
      </para>
     </listitem>
     <listitem>
      <para>
       16 divides the rank by 1 + the logarithm of the number
       of unique words in document
      </para>
     </listitem>
     <listitem>
      <para>
       32 divides the rank by itself + 1
      </para>
     </listitem>
    </itemizedlist>

    If more than one flag bit is specified, the transformations are
    applied in the order listed.
   </para>

   <para>
    It is important to note that the ranking functions do not use any global
    information, so it is impossible to produce a fair normalization to 1% or
    100% as sometimes desired.  Normalization option 32
    (<literal>rank/(rank+1)</literal>) can be applied to scale all ranks
    into the range zero to one, but of course this is just a cosmetic change;
    it will not affect the ordering of the search results.
   </para>

   <para>
    Here is an example that selects only the ten highest-ranked matches:

<screen>
SELECT title, ts_rank_cd(textsearch, query) AS rank
FROM apod, to_tsquery('neutrino|(dark &amp; matter)') query
WHERE query @@ textsearch
ORDER BY rank DESC
LIMIT 10;
                     title                     |   rank
-----------------------------------------------+----------
 Neutrinos in the Sun                          |      3.1
 The Sudbury Neutrino Detector                 |      2.4
 A MACHO View of Galactic Dark Matter          |  2.01317
 Hot Gas and Dark Matter                       |  1.91171
 The Virgo Cluster: Hot Plasma and Dark Matter |  1.90953
 Rafting for Solar Neutrinos                   |      1.9
 NGC 4650A: Strange Galaxy and Dark Matter     |  1.85774
 Hot Gas and Dark Matter                       |   1.6123
 Ice Fishing for Cosmic Neutrinos              |      1.6
 Weak Lensing Distorts the Universe            | 0.818218
</screen>

    This is the same example using normalized ranking:

<screen>
SELECT title, ts_rank_cd(textsearch, query, 32 /* rank/(rank+1) */ ) AS rank
FROM apod, to_tsquery('neutrino|(dark &amp; matter)') query
WHERE  query @@ textsearch
ORDER BY rank DESC
LIMIT 10;
                     title                     |        rank
-----------------------------------------------+-------------------
 Neutrinos in the Sun                          | 0.756097569485493
 The Sudbury Neutrino Detector                 | 0.705882361190954
 A MACHO View of Galactic Dark Matter          | 0.668123210574724
 Hot Gas and Dark Matter                       |  0.65655958650282
 The Virgo Cluster: Hot