Re: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

Robert Haas wrote:
> On Wed, Jun 15, 2016 at 1:44 PM, Kevin Grittner <kgrittn(at)gmail(dot)com>
> wrote:

> > The test I showed creates a situation which (to ANALYZE) is
> > identical to what you describe -- ANALYZE sees a page with an LSN
> > recent enough that it could have been (and actually has been)
> > pruned. Why would it be better for the ANALYZE to fail than to
> > complete?
>
> As I understand it, the reason we need to sometimes give "ERROR:
> snapshot too old" after early pruning is because we might otherwise
> give the wrong answer.

So what constitutes "the wrong answer"? A regular transaction reading a
page and not finding a tuple that should have been there but was
removed, is a serious problem and should be aborted. For ANALYZE it may
not matter a great deal. Some very old tuple that might have been
chosen for the sample is not there; a different tuple is chosen instead,
so the stats might be slightly difference. No big deal.

Maybe it is possible to get into trouble if you're taking a sample for
an expression index.

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