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

Kevin Grittner wrote:
> On Tue, May 24, 2016 at 12:00 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2016-05-24 11:24:44 -0500, Kevin Grittner wrote:
> >> On Fri, May 6, 2016 at 8:28 PM, Kevin Grittner <kgrittn(at)gmail(dot)com> wrote:
> >>> On Fri, May 6, 2016 at 7:48 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >>
> >>>> That comment reminds me of a question I had: Did you consider the effect
> >>>> of this patch on analyze? It uses a snapshot, and by memory you've not
> >>>> built in a defense against analyze being cancelled.
>
> The primary defense is not considering a cancellation except in 30
> of the 500 places a page is used. None of those 30 are, as far as
> I can see (upon review in response to your question), used in the
> analyze process.

I think what this means is that vacuum might remove tuples that would
otherwise be visible to analyze's snapshot. I suppose that's
acceptable. I wondered if it could cause harm to the size of the
sample, but after looking at acquire_sample_rows briefly I think it'd be
unharmed.

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