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

Andres Freund <andres(at)2ndquadrant(dot)com> writes:
> On 2014-07-20 18:16:51 -0400, Tom Lane wrote:
>> My point is that the cutoff multi xid won't be new enough to remove
>> non-LOCKED_ONLY (ie, post-9.3) mxids.

> Why not? Afaics this will continue to happen until multixacts are
> wrapped around once? So the cutoff multi will be new enough for that at
> some point after the pg_upgrade?

Before that happens, nextMultiXid will catch up with the minmxid = 1
values, and they'll be in the past, and then we're at the same point
that we're at to begin with if we used 9.3.5 pg_upgrade.

> Luckily in most cases full table vacuums triggered due to normal xids
> will prevent bad problems though.

Yeah. While it's not that comfortable to rely on that, we were reliant
on that effect in every pre-9.3 branch, so I'm not terribly upset about
it continuing to be the case in existing 9.3 installations. As long as
they're not consuming mxids at a rate much greater than xids, they'll be
all right; and things will be unconditionally safe once the freezing
process has advanced far enough.

regards, tom lane