Re: bug in ts_rank_cd

MY PREV EMAIL HAD A PROBLEM. Please reply to this one
======================================================

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

Sushant Sinha <sushant354(at)gmail(dot)com> writes:
> 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.

Hmm ... I cannot reproduce the behavior you're complaining of.
You say

> select ts_rank_cd(to_tsvector('english', 'abc sdd'),
> plainto_tsquery('english', 'abc'));
> ts_rank_cd
> ------------
> 0

but I get

regression=# select ts_rank_cd(to_tsvector('english', 'abc sdd'),
regression(# plainto_tsquery('english', 'abc'));
ts_rank_cd
------------
0.1
(1 row)

> The problem is that the Cover finding algorithm ignores the lexeme at
> the 0th position,

As far as I can tell, there is no "0th position" --- tsvector counts
positions from one. The only way to see pos == 0 in the input to
Cover() is if the tsvector has been stripped of position information.
ts_rank_cd is documented to return 0 in that situation. Your patch
would have the effect of causing it to return some nonzero, but quite
bogus, ranking.

regards, tom lane

Sorry for sounding the false alarm. I was not running the vanilla
postgres and that is why I was seeing that problem. Should have checked
with the vanilla one.

-Sushant

On Tue, 2010-12-21 at 23:03 -0500, Tom Lane wrote:
> Sushant Sinha <sushant354(at)gmail(dot)com> writes:
> > 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.
>
> Hmm ... I cannot reproduce the behavior you're complaining of.
> You say
>
> > select ts_rank_cd(to_tsvector('english', 'abc sdd'),
> > plainto_tsquery('english', 'abc'));
> > ts_rank_cd
> > ------------
> > 0
>
> but I get
>
> regression=# select ts_rank_cd(to_tsvector('english', 'abc sdd'),
> regression(# plainto_tsquery('english', 'abc'));
> ts_rank_cd
> ------------
> 0.1
> (1 row)
>
> > The problem is that the Cover finding algorithm ignores the lexeme at
> > the 0th position,
>
> As far as I can tell, there is no "0th position" --- tsvector counts
> positions from one. The only way to see pos == 0 in the input to
> Cover() is if the tsvector has been stripped of position information.
> ts_rank_cd is documented to return 0 in that situation. Your patch
> would have the effect of causing it to return some nonzero, but quite
> bogus, ranking.
>
> regards, tom lane