The query planner optimizes it. Why would you want a compiler to optimize SQL? The nature of your data affects how it is optimized! The declarative statement to retrieve data must be interpreted based upon the nature of that data. You can't pre-optimize without knowing something about your data, in which case, you are basically storing some of the information outside of the database.
I don't understand why anyone would prefer SQL to that for anything beyond a simple SQL query. And it's not just my opinion: industry at large uses Spark for production with complex queries. SQL is for analysts.
