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Add a rank/(rank+1) normalization option to ts_rank(). While the usefulness

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of this seems a bit marginal, if it's useful enough to be shown in the manual
then we probably ought to support doing it without double evaluation of the
ts_rank function.  Per my proposal earlier today.
This commit is contained in:
Tom Lane
2007-11-14 23:43:27 +00:00
parent 5858990f87
commit 866bad9543
2 changed files with 32 additions and 15 deletions
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22
doc/src/sgml/textsearch.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/textsearch.sgml,v 1.33 2007/11/14 18:36:37 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/textsearch.sgml,v 1.34 2007/11/14 23:43:27 tgl Exp $ -->
<chapter id="textsearch">
<title id="textsearch-title">Full Text Search</title>
@ -940,6 +940,7 @@ SELECT plainto_tsquery('english', 'The Fat &amp; Rats:C');
<listitem>
<para>
4 divides the rank by the mean harmonic distance between extents
(this is implemented only by <function>ts_rank_cd</>)
</para>
</listitem>
<listitem>
@ -953,17 +954,24 @@ SELECT plainto_tsquery('english', 'The Fat &amp; Rats:C');
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. However, a simple technique like
<literal>rank/(rank+1)</literal> can be applied. Of course, this is just
a cosmetic change, i.e., the ordering of the search results will not
change.
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>
@ -991,7 +999,7 @@ ORDER BY rank DESC LIMIT 10;
This is the same example using normalized ranking:
<programlisting>
SELECT title, ts_rank_cd(textsearch, query)/(ts_rank_cd(textsearch, query) + 1) AS rank
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;