On Sun, Jan 12, 2020 at 3:33 AM Alexander Korotkov <a.korot...@postgrespro.ru> wrote: > > On Wed, Jan 8, 2020 at 4:37 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > > Heikki Linnakangas <hlinn...@iki.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
