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Re: Inlining comparators as a performance optimisation

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Bruce Momjian <bruce(at)momjian(dot)us>, Peter Geoghegan <peter(at)2ndquadrant(dot)com>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Inlining comparators as a performance optimisation
Date: 2011-12-07 15:09:32
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Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Dec 6, 2011 at 8:46 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> 1. Adding sortsupport infrastructure for more datatypes.
>> 2. Revising nbtree and related code to use this infrastructure.
>> 3. Integrating Peter's work into this framework.
>> I'll try to take care of #1 for at least a few key datatypes before
>> I commit, but I think #2 is best done as a separate patch, so I'll
>> postpone that till later.

> I see you've committed a chunk of this now.  Does it make sense to do
> #1 for every data type we support, or should we be more selective than
> that?

Basically, I tried to do #1 for every datatype for which the comparator
was cheap enough that reducing the call overhead seemed likely to make a
useful difference.  I'm not in favor of adding sortsupport functions
where this is not true, as I think it'll be useless code and catalog
bloat.  I don't want to add 'em for cruft like abstime either.

There's some stuff that's debatable according to this criterion --- in
particular, I wondered whether it'd be worth having a fast path for
bttextcmp, especially if we pre-tested the collate_is_c condition and
had a separate version that just hardwired the memcmp code path.  (The
idea of doing that was one reason I insisted on collation being known at
the setup step.)  But it would still have to be prepared for detoasting,
so in the end I was unenthused.  Anyone who feels like testing could try
to prove me wrong about it though.

> Are you planning to do anything about #2 or #3?

I am willing to do #2, but not right now; I feel what I need to do next
is go review SPGist.  I don't believe that #2 blocks progress on #3
anyway.  I think #3 is in Peter's court, or yours if you want to do it.

(BTW, I agree with your comments yesterday about trying to break down
the different aspects of what Peter did, and put as many of them as we
can into the non-inlined code paths.)

			regards, tom lane

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