1. A machine-implemented method comprising: a computer system including one or more computers, which includes at least a processor system having at least one processor, receiving a user query; the processor system parsing the user query and automatically generating a set of parallel subqueries that in combination are equivalent to the user query in results that are expected to be returned; the computer system forming a query that is applied to multiple database tables, if a cost of the user query is expected to be more than the query formed, the forming of the query being by at least forming a clause that combines the set of parallel subqueries into a query equivalent to the user query.