On Thu, May 17, 2018 at 3:49 AM Heikki Linnakangas <hlinn...@iki.fi> wrote: > On 14/05/18 02:15, Thomas Munro wrote: > > Since commit cdf91edb (2012), nodeIndexonlyscan.c says: > > > > /* > > * Predicate locks for index-only scans must be > > acquired at the page > > * level when the heap is not accessed, since > > tuple-level predicate > > * locks need the tuple's xmin value. If we had to > > visit the tuple > > * anyway, then we already have the tuple-level lock > > and can skip the > > * page lock. > > */ > > if (tuple == NULL) > > PredicateLockPage(scandesc->heapRelation, > > > > ItemPointerGetBlockNumber(tid), > > > > estate->es_snapshot); > > > > The first sentence of that comment is no longer true as of commit > > c01262a8 (2013). As for whether it's necessary to predicate-lock the > > whole eheap page (rather than the heap tuple) anyway because of HOT > > update chains, I don't know, so I'm not sure what wording to propose > > instead. > > Hmm. If there are any updated tuples, HOT or not, the visibility map bit > will not be set, and we won't reach this code. So I think we could > acquire the tuple lock here.
Right. CC Kevin. I think we should at least fix the comment. As for changing the lock level, PredicateLockTuple() wants a heap tuple that we don't have, so we'd probably need to add a PredicateLockTid() function. -- Thomas Munro http://www.enterprisedb.com
