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From: Andres Freund <[email protected]>
To: Peter Geoghegan <[email protected]>
Cc: Tomas Vondra <[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: Thu, 14 Aug 2025 16:44:14 -0400
Message-ID: <kvyser45imw3xmisfvpeoshisswazlzw35el3fq5zg73zblpql@f56enfj45nf7> (raw)
In-Reply-To: <CAH2-WzkaTHg2X9R-gLRNBEoL82t2mkrQq-3f=y3GAzrj40fFZw@mail.gmail.com>
References: <CAH2-Wz=L7h-koDKa3_NEg39Faw7MrOkSVOsodvQ4toSQahvWjQ@mail.gmail.com>
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Hi,

On 2025-08-14 15:45:26 -0400, Peter Geoghegan wrote:
> On Thu, Aug 14, 2025 at 3:15 PM Peter Geoghegan <[email protected]> wrote:
> > Then why does the exact same pair of runs show "I/O Timings: shared
> > read=194.629" for the sequential table backwards scan (with total
> > execution time 1132.360 ms), versus "I/O Timings: shared read=352.88"
> > (with total execution time 697.681 ms) for the random table backwards
> > scan?
>
> If you're interested in trying this out for yourself, I've pushed my
> working branch here:
>
> https://github.com/petergeoghegan/postgres/tree/index-prefetch-batch-v1.2
>
> Note that the test case you'll run is added by the most recent commit:
>
> https://github.com/petergeoghegan/postgres/commit/c9ceb765f3b138f53b7f1fdf494ba7c816082aa1
>
> Run microbenchmarks/random_backwards_weird.sql to do an initial load
> of both of the tables. Then run
> microbenchmarks/queries_random_backwards_weird.sql to actually run the
> relevant queries. There are 4 such queries, but only the 2 backwards
> scan queries really seem relevant.

Interesting. In the sequential case I see some waits that are not attributed
in explain, due to the waits happening within WaitIO(), not WaitReadBuffers().
Which indicates that the read stream is trying to re-read a buffer that
previously started being read.

   read_stream_start_pending_read()
-> StartReadBuffers()
-> AsyncReadBuffers()
-> ReadBuffersCanStartIO()
-> StartBufferIO()
-> WaitIO()

There are far fewer cases of this in the random case.

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