Suppose the deferred modification technique is used in a database.

a. Is the old value part of an update log record required any more? Why or why not? 

b. If old values are not stored in update log records, transaction undo is clearly not feasible. How would the redo phase of recovery have to be modified as a result?

c. Deferred modification can be implemented by keeping updated data items in local memory of transactions and reading data items that have not been updated directly from the database buffer. Suggest how to efficiently implement a data item read, ensuring that a transaction sees its own updates.

d. What problem would arise with the above technique if transactions perform a large number of updates?