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

On 2014-07-20 13:37:01 -0400, Tom Lane wrote:
> Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > On Tue, Jul 1, 2014 at 03:01:06PM -0400, Alvaro Herrera wrote:
> >> Finally, there is the question of what to do if the database has already
> >> been upgraded and thus the tables are all at relminmxid=1. As far as I
> >> can tell, if the original value of nextMulti was below 2^31, there
> >> should be no issue because vacuuming would advance the value normally.
> >> If the original value was beyond that point, then vacuum would have been
> >> bleating all along about the wraparound point. In this case, I think it
> >> should be enough the UPDATE the pg_class values to the current
> >> oldestMulti value from pg_control, but I haven't tested this.
>
> > Well, we are already having users run a query for the 9.3.X minor
> > version upgrade to optionally remove the 0000 file. Is there something
> > else they should run to test for this? We certainly could check for
> > files >= 8000, but I am not sure that is sufficient. We would then need
> > them to somehow update all the database/relation minmxid fields, and I
> > am not even sure what value we should set it to. Is that something we
> > want to publish?
>
> I started transcribing Bruce's proposed fix procedure at
> https://wiki.postgresql.org/wiki/20140702pg_upgrade_fix
> into the release notes, but I'm afraid it's all wet.

I don't understand why we should do anything but remove the 0000 file if
it's all zeroes? This seems far too complicated. Beside the fact that I
doubt it's actually achieving anything reliably?

> I'm a bit inclined to not say anything about fix procedures in the release
> notes, because I'm not sure that this is a problem in the field. If
> anybody did have a wraparound they'd be getting bleats from VACUUM, and no
> one has reported any such thing that I've heard.

There actually have been a couple reports about the general problem I
think - reacting to one was how I noticed the bug.

Greetings,

Andres Freund

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