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

On Wed, Jun 15, 2016 at 3:25 PM, Andres Freund <andres@anarazel.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@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.

What prevents that from happening now, without STO?

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company