Re: Performance problem...

On Tue, 2005-03-15 at 02:59, Marcin Giedz wrote:
> Dnia poniedziałek, 14 marca 2005 19:32, Scott Marlowe napisał:
> > On Mon, 2005-03-14 at 12:03, Marcin Giedz wrote:
> > > Hello...
> > >
> > >
> > > Our company is going to change SQL engine from MySQL to PSQL. Of course
> > > some performance problems occured. Our server is Dual Xeon 3.0GHz + 8GB
> > > RAM + RAID1(software - two 146GB SCSI 15k) for sql data + RAID1(software
> > > - two 146GB SCSI 15k) for pg_xlog. Postgres.conf parameters are as
> > > follows:
> > >
> > > max_connections = 150
> > > shared_buffers = 50000          # min 16, at least max_connections*2, 8KB
> > > each work_mem = 2048         # min 64, size in KB
> >
> > 50,000 shared buffers may or may not be too much.  Try it at different
> > sizes from 5,000 or so up to 50,000 and find the "knee".  It's usually
> > closer to 10,000 than 50,000, but ymmv...
>
> Playing with shared_buffers from 10000 to 50000 doesn't change anything in
> total time for this query :( But when I change work_mem a little higher to
> 10000 total runtime decreases a little about 10% but when I change
> random_page_cost to 0.2 (I know that almost all papers say it should be
> higher then 1.0) total runtime decreases almost 3 times and lasts about 900ms
> - earlier with random_page_cost=1.2 it took 2.7s. Is it possible to have
> random_page_cost on this value?

IF random_page_cost needs to be that low, then it's likely that the
query planner is either getting bad statistics and making a poor
decision, or that you've got a corner case that it just can't figure
out.  What does explain analyze <yourqueryhere> say with
random_page_cost set to 1.2 and 0.2?  HAve you run analyze and vacuumed
full lately?