From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: heap_tuple_needs_freeze false positive |
Date: | 2012-02-02 16:27:19 |
Message-ID: | 25250.1328200039@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> I'm not convinced that it's a bug. Suppose that xmax is set but is
> hinted as invalid.
XMAX_INVALID is not a "hint". When it's set, the contents of the field
must be presumed to be garbage. Any code failing to adhere to that rule
is broken.
> We process the table and advanced relfrozenxid;
> then, we crash. After recovery, it's possible that the hint bit is
> gone (after all, setting hint bits isn't WAL-logged). Now we're in
> big trouble, because the next CLOG lookup on that xmax value might not
> happen until it's been reused, and we might get a different answer
> than before.
I believe we have adequate defenses against that, and even if we did
not, that doesn't make the code in question less wrong.
regards, tom lane
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