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

On 2016-07-20 13:59:32 -0400, Robert Haas wrote:
> It's hard to believe that it's equally good to use the newest
> registered snapshot (which is, I think, what you will often get from
> GetActiveSnapshot()) and the oldest registered snapshot (which is what
> you will get from pairingheap_first()). It seems to me that we need
> to analyze what happens if we choose a snapshot that is older than the
> one used to find the datum which contained the toast pointer, and
> conversely what happens if we use a snapshot that is newer than the
> one we used to find the toast pointer.

Yea, the oldest seems better.

> Here's an attempt:
>
> 1. If we pick a snapshot that is older than the one that found the
> scan tuple, we might get a "snapshot too old" error that is not
> strictly necessary.

Right. Which still seems a lot better than essentially pessimizing
vacuuming for toast tables considerably.

> 2. If we pick a snapshot that is newer than the one that found the
> scan tuple, then haven't we failed to fix the problem? I'm not so
> sure about this direction, but if it's OK to test an arbitrarily new
> snapshot, then I can't see why we need the test at all.

I think some argument could be construed why it'd possibly be safe, but
I feel a lot better with the other option.

Andres