Re: pg_upgrade libraries check

Bruce Momjian <bruce@momjian.us> writes:
> On Sun, May 27, 2012 at 08:48:54AM -0400, Andrew Dunstan wrote:
>> "things like CREATE LANGUAGE plperl" is a rather vague phrase. The
>> PL case could be easily handled by adding this to the query:
>> OR EXISTS (SELECT 1 FROM pg_catalog.pg_language WHERE lanplcallfoid
>> = p.oid)
>> Do you know of any other cases that this would miss?

Well, laninline and lanvalidator for two ;-)

> The problem is I don't know.  I don't know in what places we reference
> shared object files implicit but not explicitly, and I can't know what
> future places we might do this.

The "future changes" argument seems like a straw man to me.  We're
already in the business of adjusting pg_upgrade when we make significant
catalog changes.

> We are not writing a one-off pg_upgrade for JSON-backpatchers here.

I tend to agree with that position, and particularly think that we
should not allow the not-community-approved design of the existing
JSON backport to drive changes to pg_upgrade.  It would be better to
ask first if there were a different way to construct that backport
that would fit better with pg_upgrade.

Having said that, I've got to also say that I think we've fundamentally
blown it with the current approach to upgrading extensions.  Because we
dump all the extension member objects, the extension contents have got
to be restorable into a new database version as-is, and that throws away
most of the flexibility that we were trying to buy with the extension
mechanism.  IMO we have *got* to get to a place where both pg_dump and
pg_upgrade dump extensions just as "CREATE EXTENSION", and the sooner
the better.  Once we have that, this type of issue could be addressed by
having different contents of the extension creation script for different
major server versions --- or maybe even the same server version but
different python library versions, to take something on-point for this
discussion.  For instance, Andrew's problem could be dealt with if the
backport were distributed as an extension "json-backport", and then all
that's needed in a new installation is an empty extension script of that
name.

More generally, this would mean that cross-version compatibility
problems for extensions could generally be solved in the extension
scripts, and not with kluges in pg_upgrade.  As things stand, you can be
sure that kluging pg_upgrade is going to be the only possible fix for
a very wide variety of issues.

I don't recall exactly what problems drove us to make pg_upgrade do
what it does with extensions, but we need a different fix for them.
        regards, tom lane