Re: POC, WIP: OR-clause support for indexes

On Fri, Oct 4, 2024 at 10:24 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Interesting. I would not have guessed that. I wonder how it works.

ISTM that we've established a general expectation that you as a user
can be fairly imprecise about which specific types you use as
constants in your query, while still getting an index scan (provided
all of the types involved have opclasses that are part of the same
opfamily, and that the index uses one of those opclasses as its input
opclass). Imagine how confusing it would be if "SELECT * FROM
pgbench_accounts WHERE aid = 5" didn't get an index scan whenever the
"aid" column happened to be bigint -- that would be totally
unacceptable. The main reason why we have operator classes that are
grouped into opfamilies is to allow the optimizer to understand the
relationship between opclasses sufficient to enable this flexibility.

It's concerning that there's a performance cliff with the patch
whenever one of the constants is changed from (say) 2_147_483_647 to
2_147_483_648 -- who will even notice that they've actually mixed two
different types of integers here? Users certainly won't see any
similar problems in the simple "Var = Const" case, nor will they see
problems in the mixed-type IN() list case.

--
Peter Geoghegan