> b). How to present the element > in UniqueKey. Prue EquivalenceClasses or Mix of Expr and EquivalenceClass as > we just talked about. I think the reason we add ECs for sort expressions is to use transitive relationship. The EC may start with a single member but later in the planning that member might find partners which are all equivalent. Result ordered by one is also ordered by the other. The same logic applies to UniqueKey as well, isn't it. In a result if a set of columns makes a row unique, the set of columns represented by the other EC member should be unique. Though a key will start as a singleton it might EC partners later and thus thus unique key will transition to all the members. With that logic UniqueKey should use just ECs instead of bare expressions.
TBH, I haven't thought about this too hard, but I think when we build the
UniqueKey, all the ECs have been built already. So can you think out an
case we start with an EC with a single member at the beginning and
