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From: Thomas Munro <thomas.munro@gmail.com>
Date: Mon, 21 Jul 2025 12:59:04 +1200
Message-ID: 
 <CA+hUKGLvL408O2ss6YZQsdupryvBqjhuej_hyOYU9fobhdHkTQ@mail.gmail.com>
Subject: Re: index prefetching
To: Tomas Vondra <tomas@vondra.me>
Cc: Peter Geoghegan <pg@bowt.ie>, Andres Freund <andres@anarazel.de>,
 Robert Haas <robertmhaas@gmail.com>,
	Melanie Plageman <melanieplageman@gmail.com>,
	PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
 Georgios <gkokolatos@protonmail.com>,
	Konstantin Knizhnik <knizhnik@garret.ru>, Dilip Kumar <dilipbalaut@gmail.com>
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On Sun, Jul 20, 2025 at 1:07=E2=80=AFAM Thomas Munro <thomas.munro@gmail.co=
m> wrote:
> On Sat, Jul 19, 2025 at 11:23=E2=80=AFPM Tomas Vondra <tomas@vondra.me> w=
rote:
> > The thing that however concerns me is that what I observed was not the
> > distance getting reset to 1, and then ramping up. Which should happen
> > pretty quickly, thanks to the doubling. In my experiments it *never*
> > ramped up again, it stayed at 1. I still don't quite understand why.
>
> Huh.  Will look into that on Monday.

I suspect that it might be working as designed, but suffering from a
bit of a weakness in the distance control algorithm, which I described
in another thread[1].  In short, the simple minded algorithm that
doubles on miss and subtracts one on hit can get stuck alternating
between 1 and 2 if you hit certain patterns.  Bilal pinged me off-list
to say that he'd repro'd something like your test case and that's what
seemed to be happening, anyway?  I will dig out my experimental
patches that tried different adjustments to escape from that state....

[1] https://www.postgresql.org/message-id/flat/CA%2BhUKGLPakwZiFUa5fQXpYDpC=
XvZXQ%3DP3cWOGACCoobh7U2r3A%40mail.gmail.com