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

On Thu, Jun 16, 2016 at 1:01 PM, Andres Freund <andres@anarazel.de> wrote:

> The relevant part is the HeapTupleSatisfiesMVCC check, we're using
> SatisfiesToast for toast lookups.
>
> FWIW, I just tried to reproduce this with old_snapshot_threshold = 0 -
> but ran into the problem that I couldn't get it to vacuum anything away
> (neither main nor toast rel).   That appears to be
>                 if (old_snapshot_threshold == 0)
>                 {
>                         if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin)
>                                 && TransactionIdFollows(latest_xmin, xlimit))
>                                 xlimit = latest_xmin;
> because latest_xmin always is equal to MyPgXact->xmin, which is actually
> kinda unsurprising?

Sure -- the STO feature *never* advances the point for early
pruning past the earliest still-active transaction ID.  If it did
we would have all sorts of weird problems.

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