Horton

Re: Converting SetOp to read its two inputs separately

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

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Richard Guo <guofenglinux@gmail.com>
Cc: pgsql-hackers@lists.postgresql.org, David Rowley <dgrowleyml@gmail.com>
Date: 2024-11-20T02:09:36Z
Lists: pgsql-hackers

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Richard Guo <guofenglinux@gmail.com> writes:
> On Thu, Nov 14, 2024 at 11:00 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Aside from that minor TODO, the main thing that's left undone in this
>> patch series is to persuade the thing to exploit presorted input
>> paths.

> I think we may need to do the following to make this work:
> 1. We need to teach set_operation_ordered_results_useful() that sorted
> input paths are also useful for INTERSECT/EXCEPT, so that we can have
> setop_pathkeys set for the subqueries.
> 2. In generate_nonunion_paths(), we need to provide a valid
> "interesting_pathkeys" when calling build_setop_child_paths().

Once I'd wrapped my head around how things are done now (which the
comments in prepunion.c were remarkably unhelpful about), I saw that
most of the problem for #2 just requires re-ordering things that
generate_nonunion_paths was already doing.  As for #1, I have a modest
proposal: we should get rid of set_operation_ordered_results_useful
entirely.  It's not the code that actually does useful work, and
keeping it in sync with the code that does do useful work is hard and
unnecessary.

0001-0003 below are the same as before (so the slot-munging TODO is
still there).  0004 fixes a rather basic bug for nested set-operations
and gets rid of set_operation_ordered_results_useful along the way.
Then 0005 does your step 2.

			regards, tom lane

Commits

  1. Get rid of old version of BuildTupleHashTable().

  2. Use ExecGetCommonSlotOps infrastructure in more places.

  3. Improve planner's handling of SetOp plans.

  4. Convert SetOp to read its inputs as outerPlan and innerPlan.

  5. Fix typo in header comment for set_operation_ordered_results_useful

  6. Allow planner to use Merge Append to efficiently implement UNION