Sketching criterion for pre-filter to _view/fields-values.

I've included _keyword as well as field/value because even _keyword might run
faster in some cases if I crafted this in an awesome way.
- _expressions are no use to pre-filter in any case.
- Single fields over multiple types are also no use to pre-filter; I suspect
  _view/fields-values to run over too many unused rows. On the other hand if
  you have a bunch of very similar documents, who knows?
WHATEVER