On Fri, 8 Dec 2000, Tom Lane wrote:
> Date: Fri, 08 Dec 2000 10:47:37 -0500
> From: Tom Lane <tgl@sss.pgh.pa.us>
> To: Oleg Bartunov <oleg@sai.msu.su>
> Cc: selkovjr@mcs.anl.gov, 'pgsql-hackers ' <pgsql-hackers@postgresql.org>
> Subject: Re: [HACKERS] Indexing for geographic objects?
>
> Oleg Bartunov <oleg@sai.msu.su> writes:
> > We've done some work with GiST indices and found a little problem
> > with optimizer.
>
> > test=# set enable_seqscan = off;
> > SET VARIABLE
> > test=# explain select * from test where s @ '1.05 .. 3.95';
> > NOTICE: QUERY PLAN:
>
> > Index Scan using test_seg_ix on test (cost=0.00..369.42 rows=5000 width=12)
>
> > EXPLAIN
> > % ./bench.pl -d test -b 100 -i
> > total: 1.71 sec; number: 100; for one: 0.017 sec; found 18 docs
>
> I'd venture that the major problem here is bogus estimated selectivities
> for rtree/gist operators. Note the discrepancy between the estimated
> row count and the actual (I assume the "found 18 docs" is the true
> number of rows output by the query). With an estimated row count even
yes, 18 docs is the true number
> half that (ie, merely two orders of magnitude away from reality ;-))
> the thing would've correctly chosen the index scan over sequential.
>
> 5000 looks like a suspiciously round number ... how many rows are in
> the table? Have you done a vacuum analyze on it?
park-lane:~/app/pgsql/gist_problem$ wc SQL 10009 10049 157987 SQL
about 10,000 rows,
relevant part of script is:
.....skipped...
1.9039...3.5139
1.8716...3.9317
\.
CREATE INDEX test_seg_ix ON test USING gist (s);
vacuum analyze;
^^^^^^^^^^^^^^
explain select * from test where s @ '1.05 .. 3.95';
set enable_seqscan = off;
explain select * from test where s @ '1.05 .. 3.95';
Regards, Oleg
>
> regards, tom lane
>
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83