Re: Building infrastructure for B-Tree deduplication that recognizes when opclass equality is also equivalence

On Thu, Dec 19, 2019 at 12:05 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> My impression is that this is more of an implementation restriction
> than a design goal. I don't really remember the details, but it seems
> to me that there were locking and/or cache invalidation problems with
> making ALTER OPERATOR CLASS do more substantive things -- and that it
> was because of those problems, not a lack of desire, that we didn't
> support it.

I agree with you. My point was only that this is something that the
operator class author is really expected to get right the first time
around -- just like the behavior of B-Tree support function 1. We're
really only concerned about the upgrade path for external types that
could see a benefit from the optimization planned for nbtree (and
possibly other such optimization). Providing a non-disruptive way to
get that benefit after a pg_upgrade only seems like a nice-to-have to
me, because it's not as if anything will stop working as well as it
once did. Also, there aren't that many external types that will be
made more useful by being able to use optimizations like
deduplication; in practice almost all B-Tree indexes only use a small
handful of operator classes that are shipped in core Postgres. Once
you're using common types like text and integer, a pg_upgrade'd
database is only a REINDEX away from being able to use deduplication
(though I am not even sure if even that will be necessary in the final
patch; I hope to be able to avoid even that inconvenience with indexes
using core operator classes).

If the underlying behavior of an operator class actually changes, then
that's a disaster for all the usual reasons. It doesn't make that much
sense to reverse an earlier decision to make an operator class
BITWISE. Better to drop everything, and recreate everything, since
your indexes should be considered corrupt anyway. (Also, I don't think
that it's that hard to get it right, so this will probably never
happen.)

--
Peter Geoghegan