Re: strange pg_stats behaviour?

On Fri, Nov 29, 2002 at 08:04:15PM -0500, Tom Lane wrote:
> Hubert depesz Lubaczewski <depesz@depesz.pl> writes:
> > # explain analyze SELECT aid, foto FROM auction WHERE data_off < now() -
> > # '31 days'::interval;

first, thanks for quick reply.

> The planner doesn't know what value data_off will be compared to at
> runtime, so it has to fall back on a default selectivity estimate.

strange - is there no way we can "teach" him, that this is a constant. i
mean now() - '31 days'::interval will be exactly the same for all rows
of auction, and i belive we could put functionality info planner to
"think" about it - right side of my condition doesn't contain any call
to table fields, so it probably should be treated as constant value. or
am i totally wrong?

> Another workaround that you could look at is
> SELECT ...
> WHERE data_off <  now() - '31 days'::interval AND
>       data_off > '-infinity';

this query looks quite strange, but if it's going to help, then i think
i might use it. right now i just modify enable_seqscan before and after
my query, but this is definitelly not nice.

depesz

--
hubert depesz lubaczewski                          http://www.depesz.pl/
------------------------------------------------------------------------
Mój Boże, spraw abym milczał, dopóki się nie upewnię,  że  naprawdę  mam
coś do powiedzenia.                                      (c) 1998 depesz