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help / color / mirror / Atom feedFrom: Alexandre Felipe <[email protected]>
To: Tomas Vondra <[email protected]>
Cc: Andres Freund <[email protected]>
Cc: Peter Geoghegan <[email protected]>
Cc: Thomas Munro <[email protected]>
Cc: Nazir Bilal Yavuz <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: PostgreSQL Hackers <[email protected]>
Cc: Georgios <[email protected]>
Cc: Konstantin Knizhnik <[email protected]>
Cc: Dilip Kumar <[email protected]>
Subject: Re: index prefetching
Date: Mon, 16 Feb 2026 10:12:12 +0000
Message-ID: <CAE8JnxOzJQrb44S216Mp71dpqmsaaH5=unJB0zHgVa-+ODPMQA@mail.gmail.com> (raw)
In-Reply-To: <[email protected]>
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<CAH2-Wzm7-QuDOs6TcqfhhDsGEZCuHtn=D-SriOTnTZ_fiXNBvA@mail.gmail.com>
<CAH2-WzmH7pVQ0-mYAxb82aWbz29_BiBPq2wV5p7+1o2sRFqDRQ@mail.gmail.com>
<CAH2-Wz=6a7fGz2rALDX+xiFDuEaGQWpZ49xEaBUDKiPH8gcL+Q@mail.gmail.com>
<CAH2-WzkehuhxyuA8quc7rRN3EtNXpiKsjPfO8mhb+0Dr2K0Dtg@mail.gmail.com>
<CAH2-WzmymSyOt5Y2RGbm6cJXg18J_ttfqjdcpodHe6Gp23ConQ@mail.gmail.com>
<CAH2-Wznv9_KGqHQ1vCW2pkiA6QskBGcx5NC_-UXnD6GEQasvAQ@mail.gmail.com>
<CAE8JnxN_EwnTLLMWGhvgwaomYZ0ysm7NeogA-BqBd=Rs3S7Oqw@mail.gmail.com>
<64a2re223ajj4popowsyu4xekbuvvyfwkrihn5yzyrkwsmsuvp@2lls3tpww5dl>
<a67mvhyi2q45eg4eimhpwdg6l3s3dmpahti2svffvmvzwmss27@r4nohusvndbq>
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<[email protected]>
> How did you do that? Did you increase the number of rows, make the rows
> wider (by increasing the 'repeat' parameter in the script), or something
> else? Did you verify the table really is 1000x larger?
>
I increased the number of rows by 1000. I didn't really check the size of
the table, the time increase suggests that it was right.
> The "10k table row" means repeat('x',10000) when generating data? Oh, I
> see you're using some string_agg(), to make it not compress. But note
> that if it's TOASTed, it become entirely irrelevant for the prefetching
> test because it's in a separate relation.
>
sorry, 10k row table, for the payload I left a note
> [b] This time I used a (SELECT string_agg((i*j)::text, '+') FROM
> generate_series(1, 50)) instead of repeat('x', 100), just to prevent it
> from compressing to nothing when I try larger payloads, and hit the
> TOAST thresholds. I removed the primary key `id` because it was annoying
> to take 20 minutes to insert the data in the large scale test.
> Unfortunately, you have not included the new script, so we can't try
> reproducing your results.
>
Let me try to find something not so insane.
128kB shared buffers is a little bit ... insane. I refuse to optimize
> anything for this value, and I don't even call about regressions. Even
> 128MB is not really practical, any serious system caring about
> performance will use tens or hundreds of GBs of shared buffers.
>
I am not going to dispute that
> If the tests I am doing are pointless, should we consider having
> > something in the planner to prevent these scans from using prefetch?
> >
>
> How would you do that? Please explain.
>
I have no idea. But based on what you said I thought you would know. In my
head: "If my test seemed ridiculous to them all, they have some clear
boundaries in their mind that they could write in the planner".
> ... or you could modify the script to simply use sudo.
>
In that case sudo would request a password to the caller, and the caller is
a python script, no interaction there, of course I could do all the steps
manually, but it is more error prone (just my own mistakes are enough).
Regards,
Alexandre
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Subject: Re: index prefetching
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