(a) Consider the following relational schema: Emp(eid:integer, ename:string, age:integer, salary:real) The underlined attributes are keys for their relations. Consider the following SQL query: Select * From Emp Where age 20 and salary > 20000 Describe the index on Emp that improves the running time of this query more than every other index over most Emp relation instances. You may explain the attributes, their order, type (B+ tree or hash) of the index and whether it should be clustered