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

On 2016-07-20 11:26:11 -0400, Robert Haas wrote:
> On Wed, Jul 20, 2016 at 3:39 AM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> >>I think Snapshot's members whenTaken and lsn are updated/initialized
> >>only in GetSnapshotData(). So if GetSnapshotData() is not used, how
> >>will you expect those fields to be updated. We need those fields to
> >>be updated for TestForOldSnapshot().
> >
> > That's why I suggested copying them from the current mvcc snapshot.
>
> And how do you obtain that? The functions that reference
> SnapshotToast are toast_delete_datum, toastrel_value_exists, and
> toast_fetch_datum, toast_fetch_datum_slice, but none of those take a
> snapshot as an argument, nor is there any reasonable way to make them
> do so. Those are indirectly called by things like bttextcmp, which
> don't know what snapshot was used to fetch the datum that they are
> detoasting and can't reasonably be made to know.
>
> I mean, you could do something *approximately* correct by calling
> GetActiveSnapshot() but that doesn't seem likely to be correct in
> detail.

GetActiveSnapshot() seems like it should work well enough in this case,
or we could use pairingheap_first() to get the actual oldest registered
one.