| From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Tomas Vondra <tomas(at)vondra(dot)me>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Georgios <gkokolatos(at)protonmail(dot)com>, Konstantin Knizhnik <knizhnik(at)garret(dot)ru>, Dilip Kumar <dilipbalaut(at)gmail(dot)com> |
| Subject: | Re: index prefetching |
| Date: | 2025-08-14 19:45:26 |
| Message-ID: | CAH2-WzkaTHg2X9R-gLRNBEoL82t2mkrQq-3f=y3GAzrj40fFZw@mail.gmail.com |
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On Thu, Aug 14, 2025 at 3:15 PM Peter Geoghegan <pg(at)bowt(dot)ie> 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.
--
Peter Geoghegan
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