No, it is not a judgement call. The query planner should be storing the distributions of the values in every index. This makes it obvious which pushdown to do here. Again, basic stuff. Youre right though not quite simple as "push down as much as possible", it is one step past that.
Agreed. Isn't this precisely why key statistics (table statistics) are maintained in many DB systems? Essentially, always "push down" the predicate with the worst statistics and always execute (early) the predicates with high selectivity.
I'd be very surprised if virtually every RDBMS doesn't do this already.
Isn't their tech to address that, like golang's "singleflight"?