On Wed, 11 Jul 2018 14:37:46 +0200 Tomas Vondra <tomas.von...@2ndquadrant.com> wrote:
> > On 07/11/2018 01:28 PM, Ashutosh Bapat wrote: > > I don't think we should change add_path() for this. We will > > unnecessarily check that condition even for the cases where we do not > > create index paths. I think we should fix the caller of add_path() > > instead to add index only path before any index paths. For that the > > index list needs to be sorted by the possibility of using index only > > scan. > > > > But I think in your case, it might be better to first check whether > > there is any costing error because of which index only scan's path has > > the same cost as index scan path. Also I don't see any testcase which > > will show why index only scan would be more efficient in your case. > > May be provide output of EXPLAIN ANALYZE. > > > > I suspect this only happens due to testing on empty tables. Not only is > testing of indexes on small tables rather pointless in general, but more > importantly there will be no statistics. So we fall back to some default > estimates, but we also don't have relallvisible etc which is crucial for > estimating index-only scans. I'd bet that's why the cost estimates for > index scans and index-only scans are the same here. You are right. When the table have rows and this is vacuumed, index only scan's cost is cheaper and chosen properly. Sorry, I have jumped to the conclusion before confirming this. Thanks, -- Yugo Nagata <nag...@sraoss.co.jp>
