Re: Getting sorted data from foreign server for merge join

Attached patch with some cosmetic changes (listed here for your quick reference)
1. , was replaced with ; in comment "inner join, expressions in the " at one place, which is correct, but missed other place.

2. The comment "First, consider whether any each active EC is potentially" should use either "any" or "each". I have reworded it as "First, consider whether any of the active ECs is potentially ...". Or we can use "First, find all of the active ECs which are potentially ....".
3. "having the remote side due the sort generally won't be any worse ..." - instead of "due" we should use "do"?

4. Added static prototype of function get_useful_ecs_for_relation().
5. The comment "Extract unique EC for query, if any, so we don't consider it again." is too crisp. Phrase "Unique EC for query" is confusing; EC can not be associated with a query per say and EC's are always unique because of canonicalisation. May be we should reword it as "Extract single EC for ordering of query, if any, so we don't consider it again." Is that cryptic as well?