Re: How to avoid hashjoin and mergejoin

On 11/1/07, Carlo Stonebanks <stonec(dot)register(at)sympatico(dot)ca> wrote:
> I am comparing the same query on two different PG 8.2 servers, one Linux
> (8GB RAM) and one Windows (32GB RAM). Both have similar drives and CPU's.
>
> The Windows posgrestsql.config is pretty well tuned but it looks like
> someone had wiped out the Linux config so the default one was re-installed.
> All performance-related memory allocation values seem to be set to the
> defaults, but mods have been made: max_connections = 100 and shared_buffers
> = 32MB.
>
> The performance for this query is terrible on the Linux server, and good on
> the Windows server - presumably because the original Linux PG config has
> been lost. This query requires: that "set enable_seqscan to 'off';"

Have you run analyze on the server yet?

A few general points on performance tuning. With 8.2 you should set
shared_buffers to a pretty big chunk of memory on linux, up to 25% or
so. That means 32 Meg shared buffers is REAL low for a linux server.
Try running anywhere from 512Meg up to 1Gig for starters and see if
that helps too. Also turn up work_mem to something like 16 to 32 meg
then restart the server after making these changes.

Then give us the explain analyze output with all the enable_xxx set to ON.

summary: analyze, increase shared_buffers and work_mem, give us explain analyze.