Re: Yet another fast GiST build

From: "Andrey M(dot) Borodin" <x4mmm(at)yandex-team(dot)ru>
To: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Cc: Darafei Komяpa Praliaskouski <me(at)komzpa(dot)net>, Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Alexander Korotkov <a(dot)korotkov(at)postgrespro(dot)ru>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Yet another fast GiST build
Date: 2020-09-16 07:27:09
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> 15 сент. 2020 г., в 22:07, Heikki Linnakangas <hlinnaka(at)iki(dot)fi> написал(а):
> regression=# SELECT f1, f1 <-> '0,1' FROM point_tbl ORDER BY f1 <-> '0,1';
> f1 | ?column?
> -------------------+------------------
> (0,0) | 1
> (1e-300,-1e-300) | 1
> (-3,4) | 4.24264068711929
> (-10,0) | 10.0498756211209
> (10,10) | 13.4536240470737
> (-5,-12) | 13.9283882771841
> (5.1,34.5) | 33.885985303662
> (1e+300,Infinity) | Infinity
> (NaN,NaN) | NaN
> |
> (10 rows)
> It is arbitrary which one you get first.
> It's not very nice to have a not-well defined order of rows in the expected output, as it could change in the future if we change the index build algorithm again. But we have plenty of cases that depend on the physical row order, and it's not like this changes very often, so I think it's ok to just memorize the new order in the expected output.

I think this is valid reasoning. GiST choose subtree algorithm is not deterministic, it calls random(), but not in tested paths.
I was thinking that machine epsilon is near 1e-300, but I was wrong. It's actually near 1e-15.

Actually, I just want to understand what changes between v18 and v19 changed on-page order of items. I look into patch diff and cannot figure it out. There are only logging changes. How this affects scan?

Best regards, Andrey Borodin.

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