Re: MultiXactId error after upgrade to 9.3.4

Andrew Gierth wrote:

> But that leaves an obvious third issue: it's all very well to ignore the
> pre-upgrade (pre-9.3) mxid if it's older than the cutoff or it's in the
> future, but what if it's actually inside the currently valid range?
> Looking it up as though it were a valid mxid in that case seems to be
> completely wrong and could introduce more subtle errors.

You're right, we should not try to resolve a multixact coming from the
old install in any case.

> (It can, AFAICT, be inside the currently valid range due to wraparound,
> i.e. without there being a valid pg_multixact entry for it, because
> AFAICT in 9.2, once the mxid is hinted dead it is never again either
> looked up or cleared, so it can sit in the tuple xmax forever, even
> through multiple wraparounds.)

HeapTupleSatisfiesVacuum removes very old multixacts; see the
HEAP_IS_LOCKED block starting in line 1162 where we set
HEAP_XMAX_INVALID for multixacts that are not running by falling
through.  It's not a strict guarantee but this probably explains why
this problem is not more common.

> Why is the correct rule not "check for and ignore pre-upgrade mxids
> before even trying to fetch members"?

I propose something like the attached patch, which implements that idea.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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