Re: Add parameter jit_warn_above_fraction - Mailing list pgsql-hackers

On Fri, 8 Apr 2022 at 23:27, Magnus Hagander <magnus@hagander.net> wrote:
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> On Wed, Mar 30, 2022 at 3:04 PM Magnus Hagander <magnus@hagander.net> wrote:
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>> On Tue, Mar 29, 2022 at 10:06 PM David Rowley <dgrowleyml@gmail.com> wrote:
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>>> If we go with this patch,  the problem I see here is that the amount
>>> of work the JIT compiler must do for a given query depends mostly on
>>> the number of expressions that must be compiled in the query (also to
>>> a lesser extent jit_inline_above_cost, jit_optimize_above_cost,
>>> jit_tuple_deforming and jit_expressions). The DBA does not really have
>>> much control over the number of expressions in the query.  All he or
>>> she can do to get rid of the warning is something like increase
>>> jit_above_cost.  After a few iterations of that, the end result is
>>> that jit_above_cost is now high enough that JIT no longer triggers
>>> for, say, that query to that table with 1000 partitions where no
>>> plan-time pruning takes place.  Is that really a good thing? It likely
>>> means that we just rarely JIT anything at all!
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>> I don't agree with the conclusion of that.
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>> What the parameter would be useful for is to be able to tune those costs (or just turn it off) *for that individual query*. That doesn't mean you "rarely JIT anything atll", it just means you rarely JIT that particular query.

I just struggle to imagine that anyone is going to spend much effort
tuning a warning parameter per query.  I imagine they're far more
likely to just ramp it up to only catch some high percentile problems
or just (more likely) just not bother with it.  It seems more likely
that if anyone was to tune anything per query here it would be
jit_above_cost, since that actually might have an affect on the
performance of the query, rather than if it spits out some warning
message or not.  ISTM that if the user knows what to set it to per
query, then there's very little point in having a warning as we'd be
alerting them to something they already know about.

I looked in the -general list to see if we could get some common
explanations to give us an idea of the most common reason for high JIT
compilation time. It seems that the plans were never simple. [1] seems
due to a complex plan. I'm basing that off the "Functions: 167". I
didn't catch the full plan. From what I can tell, [2] seems to be due
to "lots of empty tables", so assuming the clamping at 1 page is
causing issues there.  I think both of those cases could be resolved
by building the costing the way I mentioned.  I admit that 2 cases is
not a very large sample size.