What is the need of a log in a DBMS? Briefly describe the various types of records that are normally present in a log.
Ans: The system log, which is generally written on stable storage, consists of the redundant data required to recover from volatile storage failures and as well from errors discovered by the transaction or the database system. System log is as well known as log and has sometimes been called he DBMS journal. It consists of the following entries (as well known as log records):
a. [start_transaction, T]: points out that transaction T has started execution.
b. [write_item, T, X, old_value, new_value]: points out that transaction T has changed the value of database item X from old_value to new_value.
C. [read_item, T, X]: points out that transaction T has read the value of database item X.
d. [commit, T]: points out that transaction T has completed successfully, and affirms that the effect of it can be committed (recorded permanently) to the database.
d. [abort, T]: points out that transaction T has been aborted.