On 21/12/2025 7:55 PM, Peter Geoghegan wrote:
> On Wed, Dec 10, 2025 at 9:21 PM Peter Geoghegan <pg@bowt.ie> wrote:
>> Attached is v4.
> Attached is v5. Changes from v4:
>
> * Simplified and optimized index-only scans, with a particular
> emphasis on avoiding regressions with nested loop joins with an inner
> index-only scan.
>
> There were quite a number of small problems/dead code related to
> index-only scans fixed by this new v5. Overall, I'm quite a bit
> happier with the state of index-only scans, which I'd not paid too
> much attention to before now.
>
> * Added Valgrind instrumentation to the hash index patch, which was
> required to fix some false positives.
>
> The generic indexam_util_batch_unlock routine had Valgrind
> instrumentation in earlier versions, just to keep nbtree's buffer
> locking checks from generating similar false positives. Some time
> later, when I added the hashgetbatch patch, there were new Valgrind
> false positives during hash index scans -- which I missed at first.
> This new v5 revisions adds similar Valgrind checks to hash itself
> (changes that add code that is more or less a direct port of the stuff
> added to nbtree by commit 4a70f829), which fixes the false positives,
> and is independently useful.
>
> The rule for amgetbatch-based index AMs is that they must have similar
> buffer locking instrumentation. That seems like a good thing.
>
> --
> Peter Geoghegan
I the previous mail I shared results of my experiments with different
prefetch distance.
I think that we should start prefetching of heap tuples not from the
second batch, but after some number of proceeded tids.
Attached please find a patch which implements this approach.
And below are updated results:
limit\prefetch on off always inc threshold
1 12074 12765 3146 3282 12394
2 5912 6198 2463 2438 6124
4 2919 3047 1334 1964 2910
8 1554 1496 1166 1409 1588
16 815 775 947 940 600
32 424 403 687 695 478
64 223 208 446 453 358
128 115 106 258 270 232
256 68 53 138 149 131
512 43 27 72 78 71
1024 28 13 38 40 38
Last column is result of prefetch with read_stream_threshold=10.
