Re: Proposal - Enabling btree_gist by default

I wrote:
> Even once that dust settles, I'm not sure that "install the extension
> by default" is an acceptable approach.

After reflecting a bit, the key problem with that is it'd break
pg_upgrade of an existing cluster that has the extension installed
normally. pg_upgrade needs to preserve OIDs of data types and some
other SQL objects, and there's no way that a pre-installed extension
would happen to match up with the OIDs the extension used before.

Maybe there's some way we could finesse that, relying on the
assumption that gbtreekey4 and siblings probably aren't being
used in user tables so their OIDs wouldn't appear on disk.
But it seems quite nontrivial to do, even if said assumption
is safe.

regards, tom lane