Re: Query performance with disabled hashjoin and mergejoin

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.

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.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us
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