Re: Improve checking for pg_index.xmin

On Sun, Jan 12, 2020 at 3:33 AM Alexander Korotkov
<a(dot)korotkov(at)postgrespro(dot)ru> wrote:
>
> On Wed, Jan 8, 2020 at 4:37 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > Heikki Linnakangas <hlinnaka(at)iki(dot)fi> writes:
> > > On 01/11/2019 01:50, Alexander Korotkov wrote:
> > >> This happens so, because we're checking that there is no broken HOT
> > >> chains after index creation by comparison pg_index.xmin and
> > >> TransactionXmin. So, we check that pg_index.xmin is in the past for
> > >> current transaction in lossy way by comparison just xmins. Attached
> > >> patch changes this check to XidInMVCCSnapshot().
> > >> With patch the issue is gone. My doubt about this patch is that it
> > >> changes check with TransactionXmin to check with GetActiveSnapshot(),
> > >> which might be more recent. However, query shouldn't be executer with
> > >> older snapshot than one it was planned with.
> >
> > > Hmm. Maybe you could construct a case like that with a creative mix of
> > > stable and volatile functions? Using GetOldestSnapshot() would be safer.
> >
> > I really wonder if this is safe at all.
> >
> > (1) Can we assume that the query will be executed with same-or-newer
> > snapshot as what was used by the planner? There's no such constraint
> > in the plancache, I'm pretty sure.
> >
> > (2) Is committed-ness of the index-creating transaction actually
> > sufficient to ensure that none of the broken HOT chains it saw are
> > a problem for the onlooker transaction? This is, at best, really
> > un-obvious. Some of those HOT chains could involve xacts that were
> > still not committed when the index build finished, I believe.
> >
> > (3) What if the index was made with CREATE INDEX CONCURRENTLY ---
> > which xid is actually on the pg_index row, and how does that factor
> > into (1) and (2)?
>
> Thank you for pointing. I'll investigate these issues in detail.
>

Are you planning to work on this patch [1] for current CF? If not,
then I think it is better to either move this to the next CF or mark
it as RWF.

[1] - https://commitfest.postgresql.org/27/2337/

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