Re: [PATCH] Erase the distinctClause if the result is unique by definition

Ashutosh Bapat <ashutosh(dot)bapat(dot)oss(at)gmail(dot)com> writes:
>> On Sat, Feb 8, 2020 at 12:53 PM Andy Fan <zhihui(dot)fan1213(at)gmail(dot)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