Re: Yet another fast GiST build

From: "Andrey M(dot) Borodin" <x4mmm(at)yandex-team(dot)ru>
To: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Cc: Michael Paquier <michael(at)paquier(dot)xyz>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, Darafei Komяpa Praliaskouski <me(at)komzpa(dot)net>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Yet another fast GiST build
Date: 2020-02-24 08:50:31
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Hi Thomas!

Thanks for looking into this! I’ll fix your notices asap.

> On 24 февр. 2020 г., at 01:58, Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
> On Thu, Feb 20, 2020 at 10:14 AM Thomas Munro <thomas(dot)munro(at)gmail(dot)com> wrote:
>> 1. We expect floats to be in IEEE format, and the sort order of IEEE
>> floats is mostly correlated to the binary sort order of the bits
>> reinterpreted as an int. It isn't in some special cases, but for this
>> use case we don't really care about that, we're just trying to
>> encourage locality.
> I suppose there is a big jump in integer value (whether signed or
> unsigned) as you cross from positive to negative floats, and then the
> sort order is reversed. I have no idea if either of those things is a
> problem worth fixing. That made me wonder if there might also be an
> endianness problem. It seems from some quick googling that all
> current architectures have integers and floats of the same endianness.
> Apparently this wasn't always the case, and some ARMs have a weird
> half-flipped arrangement for 64 bit floats, but not 32 bit floats as
> you are using here.

Yes, this leap is a problem for point as generic data type. And I do not know
how to fix it. It can cause inefficient Index Scans when searching near (0,0) and query
window touches simultaneously all quadrants (4x slower).
But everything will be just fine when all data is in 2nd quadrant.

Actually, we do not need to add this hacky code to core: we can provide colum-wise
ordering or something similar as an example.
This feature is aimed at PostGIS and they already possess bit tricks tricks [0].
I’ve taken this union code from PostGIS.


Best regards, Andrey Borodin.


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