Here's another idea: just try SELECT FOR UPDATE on the offending tuple.
As I recall, that code path has enough protections that we wouldn't try
to read the members of the multixact even if we don't see it as below
the horizon, simply by checking the infomask bits as I mentioned before.
You could try SELECT FOR UPDATE the complete table. If you had debug
symbols you could grab the specific TID that's giving you trouble from
the backtrace ... but then the table is not *that* large.
--
Álvaro Herrera http://www.2ndQuadrant.com/
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