On Thu, Feb 2, 2012 at 12:45 PM, Robert Haas wrote:
I believe the adequate defense that we have is precisely the logic you
are proposing to change.  Regardless of whether you want to call
XMAX_INVALID a hint or, say, a giant tortoise, I am fairly sure that
we don't WAL-log setting it.  That means that a bit set before a crash
won't necessarily still be set after a crash.  But the corresponding
relfrozenxid advancement will be WAL-logged, leading to the problem
scenario I described.
To put that another way, the problem isn't that we might have code
somewhere in the system that ignores HEAP_XMAX_INVALID. The problem
is that HEAP_XMAX_INVALID might not still be set on that tuple the
next time somebody looks at it, if a database crash intervenes after
that bit is set and before it is flushed to disk.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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postedFeb 2, '12 at 1:01a
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