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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
View File

@ -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;

25
src/backend/utils/adt/tsrank.c
View File

@ -7,7 +7,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/adt/tsrank.c,v 1.8 2007/09/20 18:10:57 teodor Exp $
* $PostgreSQL: pgsql/src/backend/utils/adt/tsrank.c,v 1.9 2007/11/14 23:43:27 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -25,13 +25,14 @@ static float weights[] = {0.1f, 0.2f, 0.4f, 1.0f};
#define wpos(wep) ( w[ WEP_GETWEIGHT(wep) ] )
#define RANK_NO_NORM 0x00
#define RANK_NO_NORM 0x00
#define RANK_NORM_LOGLENGTH 0x01
#define RANK_NORM_LENGTH 0x02
#define RANK_NORM_EXTDIST 0x04
#define RANK_NORM_UNIQ 0x08
#define RANK_NORM_LOGUNIQ 0x10
#define DEF_NORM_METHOD RANK_NO_NORM
#define RANK_NORM_LENGTH 0x02
#define RANK_NORM_EXTDIST 0x04
#define RANK_NORM_UNIQ 0x08
#define RANK_NORM_LOGUNIQ 0x10
#define RANK_NORM_RDIVRPLUS1 0x20
#define DEF_NORM_METHOD RANK_NO_NORM
static float calc_rank_or(float *w, TSVector t, TSQuery q);
static float calc_rank_and(float *w, TSVector t, TSQuery q);
@ -348,12 +349,17 @@ calc_rank(float *w, TSVector t, TSQuery q, int4 method)
res /= (float) len;
}
/* RANK_NORM_EXTDIST not applicable */
if ((method & RANK_NORM_UNIQ) && t->size > 0)
res /= (float) (t->size);
if ((method & RANK_NORM_LOGUNIQ) && t->size > 0)
res /= log((double) (t->size + 1)) / log(2.0);
if (method & RANK_NORM_RDIVRPLUS1)
res /= (res + 1);
return res;
}
@ -762,7 +768,7 @@ calc_rank_cd(float4 *arrdata, TSVector txt, TSQuery query, int method)
Wdoc /= (double) len;
}
if ((method & RANK_NORM_EXTDIST) && SumDist > 0)
if ((method & RANK_NORM_EXTDIST) && NExtent > 0 && SumDist > 0)
Wdoc /= ((double) NExtent) / SumDist;
if ((method & RANK_NORM_UNIQ) && txt->size > 0)
@ -771,6 +777,9 @@ calc_rank_cd(float4 *arrdata, TSVector txt, TSQuery query, int method)
if ((method & RANK_NORM_LOGUNIQ) && txt->size > 0)
Wdoc /= log((double) (txt->size + 1)) / log(2.0);
if (method & RANK_NORM_RDIVRPLUS1)
Wdoc /= (Wdoc + 1);
pfree(doc);
pfree( qr.operandexist );