| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Kevin Grittner <kgrittn(at)gmail(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(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: [HACKERS] Re: pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold < |
| Date: | 2016-06-15 22:34:40 |
| Message-ID: | 20160615223440.omhksf4senwtfc6q@alap3.anarazel.de |
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| Lists: | pgsql-committers pgsql-hackers |
On 2016-06-15 16:58:25 -0500, Kevin Grittner wrote:
> On Wed, Jun 15, 2016 at 3:25 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > On 2016-06-15 14:24:58 -0500, Kevin Grittner wrote:
> >> On Wed, Jun 15, 2016 at 2:20 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >>
> >> > We might fetch a toast tuple which
> >> > since have been re-purposed for a datum of a different type.
> >>
> >> How would that happen?
> >
> > Autovac vacuums toast and heap tables independently. Once a toast datum
> > isn't used anymore, the oid used can be reused (because it doesn't
> > conflict via GetNewOidWithIndex() anymore. If analyze then detoasts a
> > datum, which hasn't been removed, the contents of that toast id, might
> > actually be for something different.
>
> What prevents that from happening now, without STO?
Afaics we shouldn't ever look (i.e. detoast) at a "dead for everyone"
tuple in autovacuum (or anywhere else). There's one minor exception to
that, and that's enum datums in indexes, which is why we currently have
weird transactional requirements for them. I'm not entirely sure this
can be hit, but it's worth checking.
Andres
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