On 04/02/2011 15:44, Greg Smith wrote:
> Ivan Voras wrote:
>> The "vanilla" plan, with default settings is:
>
> Pause here for a second: why default settings? A default PostgreSQL
> configuration is suitable for systems with about 128MB of RAM. Since you
> say you have "good enough hardware", I'm assuming you have a bit more
> than that. The first things to try here are the list at
> http://wiki.postgresql.org/wiki/Tuning_Your_PostgreSQL_Server ; your bad
> query here looks like it might benefit from a large increase to
> effective_cache_size, and possibly an increase to work_mem as well. Your
> "bad" plan here is doing a lot of sequential scans instead of indexed
> lookups, which makes me wonder if the change in join types you're
> forcing isn't fixing that part as a coincidence.
My earlier message didn't get through so here's a repeat:
Sorry for the confusion, by "default settings" I meant "planner default
settings" not generic shared buffers, wal logs, work memory etc. - which
are adequately tuned.
> Note that the estimated number of rows coming out of each form of plan
> is off by a factor of about 200X, so it's not that the other plan type
> is better estimating anything.
Any ideas how to fix the estimates? Or will I have to simulate hints by
issuing "set enable_hashjoin=f; set enable_mergejoin=f;" for this query? :)