Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts

I wrote:
> Here's a draft patch for this. I think this will fix all cases where
> the "1" minmxid inserted by previous pg_upgrade versions is actually
> in the future at the time we run VACUUM. We would still be at risk if
> it had been in the future when pg_upgrade ran but no longer is now,
> since that would mean there could be non-lock-only mxids on disk that
> are older than "1". However, for the reasons discussed upthread, it
> seems fairly unlikely to me that people would actually get burnt in
> practice, so I'm satisfied with doing this much and no more.

Ah, belay that: as coded, that would allow truncation of clog/multixact
as soon as any one relation in any one database had sane
frozenxid/minmxid. If we want to have any pretense of being safe, we have
to postpone truncation until *all* relations have been vacuumed. So more
like the attached, instead.

regards, tom lane