Re: WIP: BRIN bloom indexes

On Fri, Oct 27, 2017 at 2:55 PM, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> wrote:
> I was rather thinking that if we can make this very robust against the
> index growing out of proportion, we should consider ditching the
> original minmax and replace it with multirange minmax, which seems like
> it'd have much better behavior.

If the multirange stuff can be done in such a way that it's just an
updated version of the same opclass, and backward-compatible on disk,
then I think this would be OK. But otherwise I don't think we ditch
what already exists. That would break upgrades via both pg_upgrade
and pg_dump, which seems like too high a price to pay to get rid of
some arguably-worse code. It's actually WORSE to drop an opclass
(which will make dumps not restore) than to do something like bump
HASH_VERSION (which doesn't affect pg_dump at all and for pg_upgrade
only requires post-upgrade steps rather than failing outright).

> I don't see any reason to put any of this in contrib.

Well, for one thing, it makes it easier to drop stuff later if we
decide we don't really want it. I think that whichever BRIN opclasses
are thought to be high quality and of general utility can go into
core, just as we've done with other index AMs. However, if we think
that something is useful-ish but maybe not something to which we want
to make a permanent commitment, putting it into contrib is good for
that.

Upgrades are easier for things in contrib, too, because there's a
built-in mechanism for people to try updating the SQL extensions
(ALTER EXTENSION .. UPDATE) and if it fails they can adjust things and
try again. When you just make a hard change to SQL definitions in a
new release, any failures that result from those changes just turn
into upgrade failures, which is IMHO a lot more painful than a failure
to update an extension version while the database is still up and
usable the whole time.

For instance, if pg_stat_activity were bundled in an extension and we
made the C code backward-compatibility with old extension versions,
then some of the upgrade pain users have had with that over the years
could have been avoided. People could update to the new version
without failures and then at their leisure try to update. If the
update failed due to dependencies, then they would have time to figure
out what to do about it and try again later; in the meantime, they'd
be on the new version.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company