Andrew Gierth wrote:
> This one came up on the IRC channel and I did some analysis there.
>
> What it seems to boil down to is that, faced with a tuple with a
> pre-upgrade mxid, we get this call path:
>
> vacuum calls heap_prepare_freeze_tuple
>
> heap_prepare_freeze_tuple calls FreezeMultiXactId
> (because HEAP_XMAX_IS_MULTI)
>
> FreezeMultiXactId checks if the mxid is older than the cutoff, but
> because this is a pre-upgrade mxid it can be literally anything, and in
> this case it's in the future not the past, so that check is skipped
>
> FreezeMultiXactId calls GetMultiXactIdMembers with allow_old=true
>
> GetMultiXactIdMembers checks for wraparound, but while it downgrades the
> error to DEBUG1 if allow_old is true and the mxid is in the past, it
> unconditionally errors if the mxid is in the future.
I was in the middle of writing up this in another thread, and your
analysis in spot on.
https://www.postgresql.org/message-id/20160615233427.GA18976@alvherre.pgsql
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Álvaro Herrera http://www.2ndQuadrant.com/
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