Re: Short-circuit sort_inner_and_outer if there are no mergejoin clauses - Mailing list pgsql-hackers

From Ashutosh Bapat
Subject Re: Short-circuit sort_inner_and_outer if there are no mergejoin clauses
Date
Msg-id CAExHW5tw6sfE3adbXwUwztPUFkGNCUVHF41rt+X1bZzYNHwCtg@mail.gmail.com
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In response to Re: Short-circuit sort_inner_and_outer if there are no mergejoin clauses  (Richard Guo <guofenglinux@gmail.com>)
Responses Re: Short-circuit sort_inner_and_outer if there are no mergejoin clauses
List pgsql-hackers


On Thu, Apr 25, 2024 at 12:50 PM Richard Guo <guofenglinux@gmail.com> wrote:

On Wed, Apr 24, 2024 at 5:13 PM Richard Guo <guofenglinux@gmail.com> wrote:
In sort_inner_and_outer, we create mergejoin join paths by explicitly
sorting both relations on each possible ordering of the available
mergejoin clauses.  However, if there are no available mergejoin
clauses, we can skip this process entirely.  It seems that this is a
relatively common scenario.  Checking the regression tests I noticed
that there are a lot of cases where we would arrive here with an empty
mergeclause_list.

FWIW, during the run of the core regression tests, I found that we enter
sort_inner_and_outer with an empty mergeclause_list a total of 11064
times.  Out of these occurrences, there are 293 instances where the join
type is JOIN_UNIQUE_OUTER, indicating the need to create a UniquePath
for the outer path.  Similarly, there are also 293 instances where the
join type is JOIN_UNIQUE_INNER, indicating the need to create a
UniquePath for the inner path.

Quickly looking at the function, the patch would make it more apparent that the function is a noop when mergeclause_list is empty. I haven't looked closely to see if creating unique path nonetheless is useful somewhere else. Please add to the next commitfest. If the patch shows some measurable performance improvement, it would become more attractive.

--
Best Wishes,
Ashutosh Bapat

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