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

On Sun, Jul 20, 2014 at 11:19:30PM -0400, Tom Lane wrote:
> 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.

Should we conclude that the multi-xact code is hopelessly complex and
needs a redesign?

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +