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

On Thu, Jun 16, 2016 at 12:14 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>> > The issue isn't there without the feature, because we (should) never
>> > access a tuple/detoast a column when it's invisible enough for the
>> > corresponding toast tuple to be vacuumed away. But with
>> > old_snapshot_timeout that's obviously (intentionally) not the case
>> > anymore. Due to old_snapshot_threshold we'll prune tuples which,
>> > without it, would still be considered HEAPTUPLE_RECENTLY_DEAD.
>>
>> Is there really an assumption that the heap and the TOAST heap are
>> only ever vacuumed with the same OldestXmin value? Because that seems
>> like it would be massively flaky.
>
> There's not. They can be vacuumed days apart. But if we vacuum the toast
> table with an OldestXmin, and encounter a dead toast tuple, by the
> definition of OldestXmin (excluding STO), there cannot be a session
> reading the referencing tuple anymore - so that shouldn't matter.

I don't understand how STO changes that. I'm not saying it doesn't
change it, but I don't understand why it would.

The root of my confusion is: if we prune a tuple, we'll bump the page
LSN, so any session that is still referencing that tuple will error
out as soon as it touches the page on which that tuple used to exist.
It won't even survive long enough to care that the tuple isn't there
any more.

Maybe it would help if you lay out the whole sequence of events, like:

S1: Does this.
S2: Does that.
S1: Now does something else.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company