There is a bug in ts_rank_cd. It does not correctly give rank when the
query lexeme is the first one in the tsvector.
Example:
select ts_rank_cd(to_tsvector('english', 'abc sdd'),
plainto_tsquery('english', 'abc'));
ts_rank_cd
------------
0
select ts_rank_cd(to_tsvector('english', 'bcg abc sdd'),
plainto_tsquery('english', 'abc'));
ts_rank_cd
------------
0.1
The problem is that the Cover finding algorithm ignores the lexeme at
the 0th position, I have attached a patch which fixes it. After the
patch the result is fine.
select ts_rank_cd(to_tsvector('english', 'abc sdd'), plainto_tsquery(
'english', 'abc'));
ts_rank_cd
------------
0.1
--- postgresql-9.0.0/src/backend/utils/adt/tsrank.c 2010-01-02 22:27:55.000000000 +0530
+++ postgres-9.0.0-tsrankbugfix/src/backend/utils/adt/tsrank.c 2010-12-21 18:39:57.000000000 +0530
@@ -551,7 +551,7 @@
memset(qr->operandexist, 0, sizeof(bool) * qr->query->size);
ext->p = 0x7fffffff;
- ext->q = 0;
+ ext->q = -1;
ptr = doc + ext->pos;
/* find upper bound of cover from current position, move up */
--
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