Consider a relation R(a,b,c,d,e) containing 6,000,000 records, where each data page of the relation holds 10 records. R is organized as a sorted file with the search key R.a. Assume that R.a is a candidate key of R, with values lying in the range 0 to 5,999,999. For the relational algebra ????????,????(????????>60,000(????)), state which of the following approaches (or combination thereof) is most likely to be the cheapest:
1. Access the sorted file for R directly.
2. Use a clustered B+ tree index on attribute R.a.
3. Use a linear hashed index on attribute R.a.
4. Use a clustered B+ tree index on attributes (R.a,R.b).
5. Use a linear hashed index on attributes (R.a,R.b).
6. Use an unclustered B+ tree index on attribute R.b.
We assume that the database considers index-only plans. Index-only plans mean that an index contains all the columns needed to answer the query without having to access the data records in the files that contain the relations in the query.