Re: [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

Wong, Yi Wen wrote:
> My interpretation of README.HOT is the check is just to ensure the chain is continuous; in which case the condition should be:
>
> > if (TransactionIdIsValid(priorXmax) &&
> > !TransactionIdEquals(priorXmax, HeapTupleHeaderGetRawXmin(htup)))
> > break;
>
> So the difference is GetRawXmin vs GetXmin, because otherwise we get the FreezeId instead of the Xmin when the transaction happened

I independently arrived at the same conclusion. Since I was trying with
9.3, the patch differs -- in the old version we must explicitely test
for the FrozenTransactionId value, instead of using GetRawXmin.
Attached is the patch I'm using, and my own oneliner test (pretty much
the same I posted earlier) seems to survive dozens of iterations without
showing any problem in REINDEX.

This patch is incomplete, since I think there are other places that need
to be patched in the same way (EvalPlanQualFetch? heap_get_latest_tid?).
Of course, for 9.4 and onwards we need to patch like you described.

This bit in EvalPlanQualFetch caught my attention ... why is it saying
xmin never changes? It does change with freezing.

/*
* If xmin isn't what we're expecting, the slot must have been
* recycled and reused for an unrelated tuple. This implies that
* the latest version of the row was deleted, so we need do
* nothing. (Should be safe to examine xmin without getting
* buffer's content lock, since xmin never changes in an existing
* tuple.)
*/
if (!TransactionIdEquals(HeapTupleHeaderGetXmin(tuple.t_data),
priorXmax))

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