Re: [HACKERS] slow count() was: tsearch2 poor performance

Magnus

On Sun, 3 Oct 2004, Magnus Hagander wrote:

> >> Hey all, its me again. If I do not do a count(product_id) on my
> >> tsearch2 queries, its actually really fast, for example;
> >>
> >
> >Hmm, I also really want to know what's the difference ?
> >Postgresql 8.0beta3 on Linux 2.4.25
> >
> >tsearchd=# explain analyze select body from txt where
> >fts_index @@ to_tsquery('oil') limit 1000;
> > QUERY PLAN
> >---------------------------------------------------------------
> >-----------------------------------------------------------------
> > Limit (cost=0.00..4027.67 rows=1000 width=315) (actual
> >time=0.053..14.662 rows=1000 loops=1)
> > -> Index Scan using fts_idx on txt (cost=0.00..12083.02
> >rows=3000 width=315) (actual time=0.049..12.552 rows=1000 loops=1)
> > Index Cond: (fts_index @@ '\'oil\''::tsquery)
> > Total runtime: 15.848 ms
> >(4 rows)
> >
> >tsearchd=# explain analyze select count(body) from txt where
> >fts_index @@ to_tsquery('oil') limit 1000;
> >
> >Didn't get result after 10 minutes :(
> >
>
> I think you're missing what LIMIT does.
>
> In the first query, it LIMITs the return from the index scan to 1000
> entries.
> In the second query, it LIMITs the return from the aggregate to 1000
> entries. The indexscan will include all matches, send then to count(),
> which returns 1 row only, which LIMIT is then applied to.
>

Thanks for explanation. I suspect this, so LIMIT is still hack

> You could probably reach the same result with a subselect:
> select count(*) FROM (select body from txt where fts_index @@
> to_tsquery('oil') limit 1000)
>

yes, timings now are equal !

>
> At least that's how I think LIMIT works. That would certainly explain
> the major time difference.
>
> //Magnus
>

Regards,
Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg(at)sai(dot)msu(dot)su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83