Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation)

On Fri, Jan 12, 2018 at 8:19 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> 1.
> + if (!IsBootstrapProcessingMode() && !indexInfo->ii_Concurrent)
> {
> - snapshot = RegisterSnapshot(GetTransactionSnapshot());
> - OldestXmin = InvalidTransactionId; /* not used */
> + OldestXmin = GetOldestXmin(heapRelation, true);
>
> I think leader and workers should have the same idea of oldestXmin for
> the purpose of deciding the visibility of tuples. I think this is
> ensured in all form of parallel query as we do share the snapshot,
> however, same doesn't seem to be true for Parallel Index builds.

Hmm. Does it break anything if they use different snapshots? In the
case of a query that would be disastrous because then you might get
inconsistent results, but if the snapshot is only being used to
determine what is and is not dead then I'm not sure it makes much
difference ... unless the different snapshots will create confusion of
some other sort.

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