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

Bruce Momjian <bruce@momjian.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.

He's suggesting copying the last checkpoint's NextMultiXactId into
datminmxid/relminmxid, which is surely the wrong thing: that's likely to
be newer than all mxids in the tables, not older than them.  I thought at
first that this was a simple thinko and he meant to write oldestMultiXid,
but here's the thing: if we're in the situation where we've got
wraparound, isn't oldestMultiXid going to be 1?  The value recorded in the
checkpoint isn't magic, it's just going to be extracted from whatever's in
pg_database; and the whole problem here is that we can't trust that data.
Where can we get a useful lower bound from?

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.

            regards, tom lane