Re: Make HeapTupleSatisfiesMVCC more concurrent

On Fri, Aug 21, 2015 at 8:22 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Wed, Aug 19, 2015 at 2:53 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
wrote:
> > I think one case where the patch can impact is for aborted transactions.
> > In TransactionIdIsInProgress(), we check for aborted transactions before
> > consulting pg_subtrans whereas with patch it will consult pg_subtrans
> > without aborted transaction check. Now it could be better to first
check
> > pg_subtrans if we found that the corresponding top transaction is
> > in-progress as that will save extra pg_clog lookup, but I just mentioned
> > because that is one difference that I could see with this patch.
> >
> > Another minor point is, I think we should modify function level comment
> > for XidInMVCCSnapshot() where it says that this applies to known-
> > committed XIDs which will no longer be true after this patch.
>
> But only if the snapshot has overflowed, right?

Yes.

> That should affect
> only a small minority of cases.
>

Agreed, but the effect would be non-negligible for such cases. One thing
I am wondering that is there any harm in calling TransactionIdDidAbort()
in slow path before calling SubTransGetTopmostTransaction(), that can
also maintain consistency of checks in both the functions?

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com