I'm surprised no answers yet, but my only suggestion is a compound index on multiple fields to cover the criteria you are looking for. I would suggest an index on
(race_date, state, sub_event, race_name)
Not to be confused with multiple indexes on the columns individually. By looking at other queries, and your table structure, the order of these fields in the index might be better served in alternate sequence, but not enough info to make that as a final call.