From: | Andres Freund <andres(at)anarazel(dot)de> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Kevin Grittner <kgrittn(at)gmail(dot)com>, Ants Aasma <ants(dot)aasma(at)eesti(dot)ee>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Noah Misch <noah(at)leadboat(dot)com>, Kevin Grittner <kgrittn(at)postgresql(dot)org>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, "pgsql-committers(at)postgresql(dot)org" <pgsql-committers(at)postgresql(dot)org> |
Subject: | Re: Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold < |
Date: | 2016-06-15 19:20:11 |
Message-ID: | 20160615192011.eicg7hwkndyx2cmo@alap3.anarazel.de |
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Lists: | pgsql-committers pgsql-hackers |
On 2016-06-15 14:50:46 -0400, Robert Haas wrote:
> On Wed, Jun 15, 2016 at 2:45 PM, Alvaro Herrera
> <alvherre(at)2ndquadrant(dot)com> wrote:
> > 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.
>
> The expression index case is the one to worry about; if there is a
> problem, that's where it is. What bothers me is that a function used
> in an expression index could do anything at all - it can read any
> table in the database.
Isn't that also a problem around fetching toast tuples? As we don't
TestForOldSnapshot_impl() for toast, We might fetch a toast tuple which
since have been re-purposed for a datum of a different type. Which can
have arbitrarily bad consequences afaics.
Andres
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