On Wed, Jul 16, 2025 at 4:40 AM Tomas Vondra <tomas@vondra.me> wrote:
> For "uniform" data set, both prefetch patches do much better than master
> (for low selectivities it's clearer in the log-scale chart). The
> "complex" prefetch patch appears to have a bit of an edge for >1%
> selectivities. I find this a bit surprising, the leaf pages have ~360
> index items, so I wouldn't expect such impact due to not being able to
> prefetch beyond the end of the current leaf page. But could be on
> storage with higher latencies (this is the cloud SSD on azure).
How can you say that the "complex" patch has "a bit of an edge for >1%
selectivities"?
It looks like a *massive* advantage on all "linear" test results.
Those are only about 1/3 of all tests -- but if I'm not mistaken
they're the *only* tests where prefetching could be expected to help a
lot. The "cyclic" tests are adversarial/designed to make the patch
look bad. The "uniform" tests have uniformly random heap accesses (I
think), which can only be helped so much by prefetching.
For example, with "linear_10 / eic=16 / sync", it looks like "complex"
has about half the latency of "simple" in tests where selectivity is
10. The advantage for "complex" is even greater at higher
"selectivity" values. All of the other "linear" test results look
about the same.
Have I missed something?
--
Peter Geoghegan
