Re: [HACKERS] Improve OR conditions on joined columns (common starschema problem)

On 13 February 2017 at 06:32, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> It's not so much poor choices as the cost of the optimization attempt ---
> if there's a K-relation OR clause, this will increase the cost of planning
> by a factor approaching K+1, whether or not you get a better plan out of
> it.  I ran the regression tests with some instrumentation and determined
> that this logic fires a dozen or two times, and fails to produce a plan
> that looks cheaper than the standard plan in any of those cases.  So if we
> go down this road, not only do we need a GUC but I suspect it had better
> default to off; only people using star schemas are really likely to get a
> win out of it.

I always try to shy away from assuming that the regression test suite
is a good reflection of a real world set of queries. It's full of
tests for edge cases that are rarely seen in reality. FWIW I did a
similar experiment with unique joins and was disappointed to see that
it didn't apply in more cases. Yet I've worked with OLTP applications
since 2005, and I struggle to recall any many:many joins at all.

Perhaps this optimisation is a candidate for only being applied when
some sort of planner_strength GUC (as mentioned in FOSDEM developer
meeting in 2016) reaches some threshold. There's certainly already
some planner smarts that can be skipped when such a GUC is set to a
lower level (e.g join removal). We could likely save many cycles if we
had the ability to re-plan queries where total_cost > X with more
smarts enabled.

-- David Rowley                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services