On Fri, 2 May 2025 at 14:44, Bruce Momjian <bruce@momjian.us> wrote:
> You can see the most current HTML-built version here:
>
> https://momjian.us/pgsql_docs/release-18.html
Thanks for working on these.
For "Improve the performance of hash joins (David Rowley)", 0f5738202
did the same thing for GROUP BY and hashed subplans too. It might be
worth adjusting this to some more generic text which covers all of
these. Something like "Speed up hash value generation in Hash Join,
GROUP BY, hashed Subplan and hashed set operations</p><p>This change
also allows JIT compilation for obtaining hash values for these
operations". The set operations I likely should have mentioned in the
commit message.
There's also Jeff's work in cc721c459, 4d143509c, a0942f441, 626df47ad
which does work to reduce the memory overheads of hashed GROUP BY,
hashed Subplans and hashed set operations. I think Jeff might have
understated the possible performance gains from these commits. I very
much think this is worth something like "Reduce memory overheads for
hashed GROUP BY, subplans and set operation processing (Jeff Davis)".
A quick test with: explain analyze select a from
generate_series(1,1000000) a group by a;
v17: Batches: 1 Memory Usage: 90145kB
v18: Batches: 1 Memory Usage: 57385kB
A 37% reduction for this case. Not insignificant.
David
