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

On 2016-06-15 14:24:58 -0500, Kevin Grittner wrote:
> On Wed, Jun 15, 2016 at 2:20 PM, Andres Freund <andres@anarazel.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.

That's not super likely to happen (given how rare oid wraparounds
usually are), but it appears to be possible.

Regards,

Andres