Re: join_collapse_limit = 14

På lørdag 07. januar 2017 kl. 17:48:49, skrev Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us
<mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us>>:
Andreas Joseph Krogh <andreas(at)visena(dot)com> writes:
> I wonder; In general, is there any downside of having join_collapse_limit =
14
> on modern hardware (32 cores, 64GB RAM), and geqo_threshold=16 ?
> I'm aware of it increasing planning-time, but is this really an issue in
> practice?

It can be.  The number of possible join orders is exponential in the join
size (at least 2^n, maybe worse, don't recall at the moment).  There are
heuristics in the planner that usually let it avoid investigating large
parts of that space; but the only thing that will certainly limit the
problem is preventing large join subproblems from being considered in the
first place --- which is what join_collapse_limit/from_collapse_limit are
for.

Depending on what your queries are like, you might not ever hit any of the
bad cases, so it might be fine.  But I'd be nervous about throwing a wide
mix of complex queries at a planner set with high thresholds.

If you've got just one problem query, it might be worth your time to take
note of the optimal join order (as seen in EXPLAIN when a good plan is
chosen) and rearrange the query so that the syntactic join order matches
that.  Then you should get the same plan even when join_collapse_limit is
small.
 
Thanks.
Will geqo kick if if join_collapse_limit =4 and there are many more joins or
will the planner just stop trying to rearrange JOINs after this limit?
In other words; Is join_collapse_limit=1 the only way or might it work for
other "small" values as well?
 
In any case; I assume you're suggesting doing something like "SET LOCAL
join_collapse_limit=<some-value>" in the same transaction so this doesn't have
to be a system-wide setting?
 
-- Andreas Joseph Krogh
CTO / Partner - Visena AS
Mobile: +47 909 56 963
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