total unique values for url.orginal or total possible unique values for all possible results when using
The thing about prefix/infix queries and keyword fields is the index is of limited use to you.
Unlike an exact-match query or leading wildcard, the alphabetic sorting of the list of unique terms can't be used to quickly seek to the relevant part - infix/prefix queries have to scan the full list of all unique terms.
With URLs I imagine while there's a lot of unique values they're not evenly divided. So quite "Zipfy" e.g. a handful of URLs account for 90% of all mentions. These very popular URLs will be slow to query with
wildcard field because it has to verify each use of a term in a doc whereas
keyword fields need only find the term in the index and be certain all docs listed with that term genuinely do have that term.
Benchmarking is the only way to know for sure which is the best approach as so much depends on your data.