Re: index prefetching - Mailing list pgsql-hackers
1) v12-0001-Use-simple-hash-for-PrivateRefCount.patch
- seems like a (mostly) mechanical switch for a different hashtable
But Andres slapped me with a question: "Did you try on a real query?"
When I tried on a real query it was a bit slower on the that Andres gave as an
example:
SELECT sum(balance) INTO s FROM uncorrelated WHERE id > 1000 and id < 500000;
It is marginal but consistently slower, I guess it is just the presence of more code
patched: 120.36 - 124.98
master: 116.19 - 118.09
Passing -falign-functionns=64
patched: 117.82 - 118.88
master: 115.40 - 119.91Maybe just the presence of more the code, because we don't touch the hash.
I tried with SH_SCOPE `static` or `static pg_noinline` but that doesn't help either.
Assuming one ReadBuffer/ReleaseBuffer per row, we can infer infer that it was
~3ms / 5e5 = 6ns slower.
On micro benchmarking it cuts the gap by half when holding 100 to 1000 buffers.
If we can accept one more pointer dereference, by having a pointer instead of a static
array we can reduce by the ramp by 80% (and no, that is not achieved by placing
the bottom of the ramp half way up hehe)
Regards,
Alexandre
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