On Wed, Jun 5, 2013 at 6:56 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> Now, I did find a couple that I thought should probably stick with
>> SnapshotNow, specifically pgrowlocks and pgstattuple. Those are just
>> gathering statistical information, so there's no harm in having the
>> snapshot change part-way through the scan, and if the scan is long,
>> the user might actually regard the results under SnapshotNow as more
>> accurate. Whether that's the case or not, holding back xmin for those
>> kinds of scans does not seem wise.
>
> FWIW, I think if we're going to ditch SnapshotNow we should ditch
> SnapshotNow, full stop, even removing the tqual.c routines for it.
> Then we can require that *any* reference to SnapshotNow is replaced by
> an MVCC reference prior to execution, and throw an error if we actually
> try to test a tuple with that snapshot. If we don't do it like that
> I think we'll have errors of omission all over the place (I had really
> no confidence in your original patch because of that worry). The fact
> that there are a couple of contrib modules for which there might be an
> arguable advantage in doing it the old way isn't sufficient reason to
> expose ourselves to bugs like that. If they really want that sort of
> uncertain semantics they could use SnapshotDirty, no?
I had the same thought, initially. I went through the exercise of
doing a grep for SnapshotNow and trying to eliminate as many
references as possible, but there were a few that I couldn't convince
myself to rip out. However, if you'd like to apply the patch and grep
for SnapshotNow and suggest what to do about the remaining cases (or
hack the patch up yourself) I think that would be great. I'd love to
see it completely gone if we can see our way clear to that.
--
Robert Haas
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