just a note here ... recently, we had a client with similar problems with
using index scan, where turning off seqscan did the trick ... we took his
tables, loaded them into a v7.1beta1 server and it correctly comes up with
the index scan ...
Oleg, have you tried this with v7.1 yet?
On Fri, 8 Dec 2000, Tom Lane wrote:
> 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
> 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?
>
> regards, tom lane
>
Marc G. Fournier ICQ#7615664 IRC Nick: Scrappy
Systems Administrator @ hub.org
primary: scrappy@hub.org secondary: scrappy@{freebsd|postgresql}.org
