C periodic checkpoints decreased the number of log entries that need to be redone during recovery

　　D during checkpointing, updated disk blocks are written to disk

　　E none of the above

　　13 There is a huge relation R(A,B,C) of size M and A is the primary key. The tuples in R are sorted by using A and no index is built for R. Each disk block size of b, and each block read takes N seconds. To find a tuple in R with A =17954 the estimated optimized cost is

　　A M*N/b

　　B N*log(M/b)

　　C M*N/(2b)

　　D (M+N)/b

　　E log(M*N)/b

　　14 Which of the following descriptions of reader/writer locks are/is wrong?

　　A caller cannot upgrade its read lock(already obtained) to write lock

　　B caller can downgrade its write lock(already obtained) to read lock

　　C caller can get read lock if there is no write lock held by others

　　D multiple callers can hold a read lock at the same time

　　E only one caller can hold a write lock at a time