Adding Subqueries to MySQL, or What Does it Take to Have a Decision-Support Engine? *

Abstract

MySQL is an OLTP system without the ability to handle SQL subqueries. As part of our project in query optimization , we add subquery processing to MySQL through rewriting. First, we incorporate the ability to handle subqueries in the FROM clause; then rewrite queries with subqueries in the WHERE clause by moving the subquery to the FROM clause and introducing suitable conditions to link the subquery and the main query tables. The result is a complete and correct unnesting procedure for SQL queries. The idiosyncrasies of SQL semantics make some rewritings tricky. While optimization issues are not directly addressed, the rewriting opens up the possibility of more efficient query processing in several cases.