Re: Indexing for geographic objects?

On Fri, 8 Dec 2000, The Hermit Hacker wrote:

> Date: Fri, 8 Dec 2000 12:19:56 -0400 (AST)
> From: The Hermit Hacker <scrappy@hub.org>
> To: Tom Lane <tgl@sss.pgh.pa.us>
> Cc: Oleg Bartunov <oleg@sai.msu.su>, selkovjr@mcs.anl.gov,
>     'pgsql-hackers ' <pgsql-hackers@postgresql.org>
> Subject: Re: [HACKERS] Indexing for geographic objects? 
> 
> 
> 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?  

Not yet. Just a plain 7.0.3 release. Will play with 7.1beta.
But we're working in real life and need things to work in production :-)
regards,    Oleg

> 
> 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 
> 

_____________________________________________________________
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