Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Dec 4, 2012 at 3:38 PM, Jeff Davis <pgsql@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.
regards, tom lane