Re: Problem with default partition pruning

Hello,

On 2019-Aug-06, Amit Langote wrote:

> On Mon, Aug 5, 2019 at 11:39 PM Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:

> > I don't think that we care about what happens with constraint_exclusion
> > is on. That's not the recommended value for that setting anyway.
>
> One issue I expressed with unconditionally applying constraint
> exclusion in partprune.c the way the third patch proposes to do it is
> that it means we end up performing the same processing twice for a
> given relation in come cases. Specifically, when constraint_exclusion
> is set to on, relation_excluded_by_constraints() that occurs when
> set_rel_sizes() is applied to that relation would perform the same
> proof. But maybe we shouldn't worry about the repetition too much,
> because it's not likely to be very problematic in practice,
> considering that setting constraint_exclusion to on is not
> recommended.

Well, if this is really all that duplicative, one thing we could do is
run this check in get_partprune_steps_internal only if
constraint_exclusion is a value other than on; we should achieve the
same effect with no repetition. Patch for that is attached. However,
if I run the server with constraint_exclusion=on, the regression test
fail with the attached diff. I didn't look at what test is failing, but
it seems to me that it's not really duplicative in all cases, only some.
Therefore we can't do it.

--
Álvaro Herrera https://www.2ndQuadrant.com/
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