Horton

Re: why partition pruning doesn't work?

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>, Pavel Stehule <pavel.stehule@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2018-06-01T15:53:39Z
Lists: pgsql-hackers 
Ashutosh Bapat <ashutosh.bapat@enterprisedb.com> writes:
> I think the patch is right if we were to handle only SQLValueFunction,
> but the bigger picture here is that we aren't evaluating stable
> functions before run-time partition pruning happens. I was under the
> impression that the stable functions/expressions get evaluated and
> folded into a constant just before the execution begins since a stable
> function produces the same output for same input during one execution
> invocation. But I am not able to find where we do that and probably we
> don't do that at all.

We don't; there was a patch floating around to make that happen, but
it hasn't been updated lately.

I agree though that it seems strange to special-case SQLValueFunction
rather than any-stable-expression.  As long as the evaluation happens
at executor start (i.e. with the query's run-time snapshot) it should
be reasonable to simplify any stable expression.

It's worth questioning whether this is a bug fix or an improvement.
If the latter, it probably ought to wait for v12.

			regards, tom lane

Commits

  1. Fix up run-time partition pruning's use of relcache's partition data.

  2. Fix access to just-closed relcache entry.

  3. Improve ExecFindInitialMatchingSubPlans's subplan renumbering logic.

  4. Improve commentary about run-time partition pruning data structures.

  5. Fix run-time partition pruning code to handle NULL values properly.

  6. Assorted cosmetic cleanup of run-time-partition-pruning code.

  7. Relocate partition pruning structs to a saner place.

  8. Improve run-time partition pruning to handle any stable expression.

  9. Support partition pruning at execution time