E none of the above

　　11 There are two relations R1(A, B, C) and R2(B, D, E). Which of following techniques CANNOT be used to improve the performance of the natural join of the two relations?

　　A make use of multiple parallel processes on a multi-core machine to implement the join algorithm

　　B increase the CPU L2 cache hit ratio of the join algorithm implementation

　　C partition the relations to different machines by using join attributes and execute the join on many machines in parallel

　　D when the number of tuples in R2 is large and the number of tuples in R1 is small, build and index on R1 and use index join method

　　E none of the above

　　12 Which of the following statements about checkpoints(with log-based recovery schemes) in databases are/is NOT correct?

　　A checkpoints are used to decrease the time used for failure recovery

　　B checkpoints are done periodically in database to assure ACID properties of transactions