Ashutosh Bapat <ashutosh.bapat....@gmail.com> writes: >> On Sat, Feb 8, 2020 at 12:53 PM Andy Fan <zhihui.fan1...@gmail.com> wrote: >> Do you mean adding some information into PlannerInfo, and when we create >> a node for Unique/HashAggregate/Group, we can just create a dummy node?
> Not so much as PlannerInfo but something on lines of PathKey. See PathKey > structure and related code. What I envision is PathKey class is also > annotated with the information whether that PathKey implies uniqueness. > E.g. a PathKey derived from a Primary index would imply uniqueness also. A > PathKey derived from say Group operation also implies uniqueness. Then just > by looking at the underlying Path we would be able to say whether we need > Group/Unique node on top of it or not. I think that would make it much > wider usecase and a very useful optimization. FWIW, that doesn't seem like a very prudent approach to me, because it confuses sorted-ness with unique-ness. PathKeys are about sorting, but it's possible to have uniqueness guarantees without having sorted anything, for instance via hashed grouping. I haven't looked at this patch, but I'd expect it to use infrastructure related to query_is_distinct_for(), and that doesn't deal in PathKeys. regards, tom lane