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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: Wed, 03 Jul 2019 15:20:15 -0400
Message-ID: <2305.1562181615@sss.pgh.pa.us> (raw)
In-Reply-To: <CAKJS1f9RUaLrujzpqf15gg_7Wo9-ai7b3-MR=bMH7a-pOWdeZA@mail.gmail.com>
References: <481.1551390571@sss.pgh.pa.us>
	<24783.1551568303@sss.pgh.pa.us>
	<20190303043424.itzy3ge52xrkpmpr@alap3.anarazel.de>
	<437.1551637744@sss.pgh.pa.us>
	<CA+TgmoZM4c=v0ifR++i09JFtNtnqTq_HkFmbFjwWqbUJffOLmA@mail.gmail.com>
	<12684.1551723095@sss.pgh.pa.us>
	<20190304190612.vgqqsowzkrh22623@alap3.anarazel.de>
	<26464.1551734920@sss.pgh.pa.us>
	<20190304221101.hdg4vj5fo4eewh3b@alap3.anarazel.de>
	<CAKJS1f-pNfoUJrmU8vgD1WrKLqs9WxOVLQ0RpSWu9h6W9RnypA@mail.gmail.com>
	<20190304235402.nod3gbotk2qtd4nh@alap3.anarazel.de>
	<CAKJS1f-yS201hAAmnxcLb2aeTYNZ_A9R1JNXF6hTVXjhHYN7rQ@mail.gmail.com>
	<1131.1551746172@sss.pgh.pa.us>
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	<29297.1558799327@sss.pgh.pa.us>
	<f078ce63-9e04-0f3e-d200-d7ee66279abe@redhat.com>
	<25178.1562006685@sss.pgh.pa.us>
	<25258.1562023641@sss.pgh.pa.us>
	<CAKJS1f9RUaLrujzpqf15gg_7Wo9-ai7b3-MR=bMH7a-pOWdeZA@mail.gmail.com>

David Rowley <david.rowley@2ndquadrant.com> writes:
> I also did some more benchmarking of the patch. ...
> Which makes the patched version 2.2% faster than master on that run.

BTW, further on the subject of performance --- I'm aware of at least
these topics for follow-on patches:

* Fix places that are maintaining arrays parallel to Lists for
access-speed reasons (at least simple_rte_array, append_rel_array,
es_range_table_array).

* Look at places using lcons/list_delete_first to maintain FIFO lists.
The patch makes these O(N^2) for long lists.  If we can reverse the list
order and use lappend/list_truncate instead, it'd be better.  Possibly in
some places the list ordering is critical enough to make this impractical,
but I suspect it's an easy win in most.

* Rationalize places that are using combinations of list_copy and
list_concat, probably by inventing an additional list-concatenation
primitive that modifies neither input.

* Adjust API for list_qsort(), as discussed, to save indirections and
avoid constructing an intermediate pointer array.  I also seem to recall
one place in the planner that's avoiding using list_qsort by manually
flattening a list into an array, qsort'ing, and rebuilding the list :-(

I don't think that any one of these fixes would move the needle very
much on "typical" simple workloads, but it's reasonable to hope that in
aggregate they'd make for a noticeable improvement.  In the meantime,
I'm gratified that the initial patch at least doesn't seem to have lost
any ground.

			regards, tom lane





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