On Fri, May 9, 2025 at 9:42 AM Peter Geoghegan <pg@bowt.ie> wrote:
> I'm rather puzzled as to why this happens, then. I expect that nbtree
> preprocessing will be able to use its usual single index column/index
> key fast path here -- the "We can short-circuit most of the work if
> there's just one key" path in _bt_preprocess_keys (and I expect that
> _bt_num_array_keys() quickly determines that no skip arrays should be
> added, preventing array preprocessing from ever really starting).
You've been testing commit 92fe23d9 ("Add nbtree skip scan
optimization") here, but I think you should test commit 8a510275
("Further optimize nbtree search scan key comparisons") instead. The
former commit's commit message says that there big regressions, that
the latter commit should go on to fix. Note that these two commits
were pushed together, as a unit. All of my performance validation work
was for the patch series as a whole, not for any individual commit.
I don't actually think that this kind of scan would have been affected
by those known regressions -- since they don't use array keys. But it
is definitely true that the queries that you're looking at very much
rely on the optimization from commit 8a510275 (or its predecessor
optimization, the "pstate.prechecked" optimization). As I said, my
performance validation didn't target individual commits.
--
Peter Geoghegan
