On 6 December 2012 00:43, Tom Lane <t...@sss.pgh.pa.us> wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Tue, Dec 4, 2012 at 3:38 PM, Jeff Davis <pg...@j-davis.com> wrote: >>> After reading that thread, I still don't understand why it's unsafe to >>> set HEAP_XMIN_COMMITTED in those conditions. Even if it is, I would >>> think that a sufficiently narrow case -- such as CTAS outside of a >>> transaction block -- would be safe, along with some slightly broader >>> cases (like BEGIN; CREATE TABLE; INSERT/COPY). > >> I haven't looked at the committed patch - which seemed a bit >> precipitous to me given the stage the discussion was at - but I >> believe the general issue with HEAP_XMIN_COMMITTED is that there might >> be other snapshots in the same transaction, for example from open >> cursors. > > From memory, the tqual.c code assumes that any tuple with XMIN_COMMITTED > couldn't possibly be from its own transaction, and thus it doesn't make > the tests that would be appropriate for a tuple that is from the current > transaction. Maybe it's all right anyway (i.e. if we should always treat > such a tuple as good) but I don't recall exactly what's tested in those > paths.
Yes. We'd need to add in a call to TransactionIdIsCurrentTransactionId(xmin), which is basically just wasted path in 99% of use cases. I've looked at optimising TransactionIdIsCurrentTransactionId() with what appears some vague success. Attached patch gives more consistent response. The other thing to do is to have a backend local flag that gets set when we use the HEAP_XMIN_COMMITTED anywhere. When not set we just skip past the TransactionIdIsCurrent test altogether. -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
optimize_transactionidiscurrent.v1.patch
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