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From: Tom Lane <tgl@sss.pgh.pa.us>
To: David Rowley <david.rowley@2ndquadrant.com>
Cc: Jesper Pedersen <jesper.pedersen@redhat.com>
Cc: Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: POC: converting Lists into arrays
Date: Mon, 22 Jul 2019 00:37:36 -0400
Message-ID: <29072.1563770256@sss.pgh.pa.us> (raw)
In-Reply-To: <CAKJS1f_OgANSMp+W_FXJWCN3yLMCE_pLfv-Y-U16vwaPtgL0Og@mail.gmail.com>
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David Rowley <david.rowley@2ndquadrant.com> writes:
> On Mon, 22 Jul 2019 at 02:45, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> One small question is whether it loses if most of the subplans
>> are present in the bitmap.  I imagine that would be close enough
>> to break-even, but it might be worth trying to test to be sure.

> ...
> -- Test 2 (all matching subplans (8192 of them)) --

> Master version:

> Time: 14825.304 ms (00:14.825)
> Time: 14701.601 ms (00:14.702)
> Time: 14650.969 ms (00:14.651)

> Patched version:

> Time: 44551.811 ms (00:44.552)
> Time: 44357.915 ms (00:44.358)
> Time: 43454.958 ms (00:43.455)

> So the bms_next_member() loop is slower when the bitmapset is fully
> populated with all subplans, but way faster when there's just 1
> member.

Interesting.  I wonder if bms_next_member() could be made any quicker?
Still, I agree that this is negligible compared to the actual work
needed per live subplan, and the fact that the cost scales per live
subplan is a good thing.  So no objection to this patch, but a mental
note to take another look at bms_next_member() someday.

			regards, tom lane





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