Re: Query with large number of joins

Marco Di Cesare <Marco.DiCesare@pointclickcare.com> writes:
> We are using a BI tool that generates a query with an unusually large number of joins. My understanding is that with
thismany joins Postgres query planner can't possibly use an exhaustive search so it drops into a heuristics algorithm.
Unfortunately,the query runs quite slow (~35 seconds) and seems to ignore using primary keys and indexes where
available.

> Query plan here (sorry had to anonymize):
> http://explain.depesz.com/s/Uml

It's difficult to make any detailed comments when you've shown us only an
allegedly-bad query plan, and not either the query itself or the table
definitions.

However, it appears to me that the query plan is aggregating over a rather
large number of join rows, and there are very few constraints that would
allow eliminating rows.  So I'm not at all sure there is a significantly
better plan available.  Are you claiming this query was instantaneous
on SQL Server?

The only thing that jumps out at me as possibly improvable is that with
a further increase in work_mem, you could probably get it to change the
last aggregation step from Sort+GroupAggregate into HashAggregate,
which'd likely run faster ... assuming you can spare some more memory.

            regards, tom lane