Re: UNION ALL on partitioned tables won't use indices.

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Noah Misch <noah(at)leadboat(dot)com>
Cc: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>, peter_e(at)gmx(dot)net, robertmhaas(at)gmail(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: UNION ALL on partitioned tables won't use indices.
Date: 2014-03-03 19:05:10
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Noah Misch <noah(at)leadboat(dot)com> writes:
> If you are convinced that a separate flattening pass is best, that suffices
> for me at this stage. Please submit the patch you have in mind, incorporating
> any improvements from the v7 patch that are relevant to both approaches.

I went back and re-read the original message, and this time it struck me
that really the issue here is that add_child_rel_equivalences() doesn't
think it has to deal with the case of a "parent" rel being itself a child.
That's not inherent though; it's just that it didn't occur to me at the
time that such a situation could arise. It takes only very small changes
to allow that to happen.

If you do that, as in the attached changes to equivclass.c, you get
"could not find pathkey item to sort" errors from createplan.c; but
that's just because create_merge_append_plan() is likewise not expecting
the mergeappend's parent rel to be itself a child. Fix for that is
a one-liner, ie, pass down relids.

That gets you to a point where the code generates a valid plan, but it's
got nested MergeAppends, which are unnecessary expense. Kyotaro-san's
original fix for that was overcomplicated. It's sufficient to teach
accumulate_append_subpath() to flatten nested MergeAppendPaths.

In short, the attached patch seems to fix it, for a lot less added
complication than anything else that's been discussed on this thread.
I've only lightly tested it (it could use a new regression test case),
but unless someone can find a flaw I think we should use this approach.

regards, tom lane

Attachment Content-Type Size
support-nested-mergeappends.patch text/x-diff 4.2 KB

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