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11c1239881
Pengchengliu reported an assertion failure in a parallel woker while performing a parallel scan using an overflowed snapshot. The proximate cause is that TransactionXmin was set to an incorrect value. The underlying cause is incorrect snapshot handling in parallel.c. In particular, InitializeParallelDSM() was unconditionally calling GetTransactionSnapshot(), because I (rhaas) mistakenly thought that was always retrieving an existing snapshot whereas, at isolation levels less than REPEATABLE READ, it's actually taking a new one. So instead do this only at higher isolation levels where there actually is a single snapshot for the whole transaction. By itself, this is not a sufficient fix, because we still need to guarantee that TransactionXmin gets set properly in the workers. The easiest way to do that seems to be to install the leader's active snapshot as the transaction snapshot if the leader did not serialize a transaction snapshot. This doesn't affect the results of future GetTrasnactionSnapshot() calls since those have to take a new snapshot anyway; what we care about is the side effect of setting TransactionXmin. Report by Pengchengliu. Patch by Greg Nancarrow, except for some comment text which I supplied. Discussion: https://postgr.es/m/002f01d748ac$eaa781a0$bff684e0$@tju.edu.cn |
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