Re: query planner not using the correct index

"Joshua Shanks" <jjshanks(at)gmail(dot)com> writes:

> Those 3 values in reality and in the stats account for 98% of the
> rows. actual distinct values are around 350

Measuring n_distinct from a sample is inherently difficult and unreliable.
When 98% of your table falls into those categories it's leaving very few
chances for the sample to find many other distinct values.

I haven't seen the whole thread, if you haven't tried already you could try
raising the statistics target for these columns -- that's usually necessary
anyways when you have a very skewed distribution like this.

> It seems like the planner would want to get the result set from
> bars.bars_id condition and if it is big using the index on the join to
> avoid the separate sorting, but if it is small (0-5 rows which is our
> normal case) use the primary key index to join and then just quickly
> sort. Is there any reason the planner doesn't do this?

Yeah, Heikki's suggested having a kind of "branch" plan node that knows how
where the break-point is between two plans and can call the appropriate one.
We don't have anything like that yet.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's On-Demand Production Tuning