1. (a) In an environment in which several users share access to a single database, can one user ever block anothers access for an unlimited period of time? (This situation is called indefinite postponement.) (b) Describe a scenario in which two users could cause the indefinite postponement of each other. (c) Describe a scenario in which a single user could cause the indefinite postponement of all users. 2. Using the two-step commit presented in the beginning of this chapter, describe how to avoid assigning one seat to two people, as in the airline example. That is, list precisely which steps the database manager should follow in assigning passengers to seats. 3. Suppose a database manager were to allow nesting of one transaction inside another. That is, after having updated part of one record, the DBMS would allow you to select another record, update it, and then perform further updates on the first record. What effect would nesting have on the integrity of a database? Suggest a mechanism by which nesting could be allowed. 4. Can a database contain two identical records without a negative effect on the integrity of the database? Why or why not? 5. Some operating systems perform buffered I/O. In this scheme, an output request is accepted from a user and the user is informed of the normal I/O completion. However, the actual physical write operation is performed later, at a time convenient to the operating system. Discuss the effect of buffered I/O on integrity in a DBMS. 6. A database transaction implements the command set STATUS to CURRENT in all records where BALANCE-OWED = 0. (a) Describe how that transaction would be performed with the two-step commit described in this chapter. (b) Suppose the relations from which that command was formed are (CUSTOMER-ID,STATUS) and (CUSTOMER-ID,BALANCE-OWED). How would the transaction be performed? (c) Suppose the relations from which that command was formed are (CUSTOMER-ID,STATUS), (CREDIT-ID,CUSTOMER-ID), (CREDIT-ID, BALANCE-OWED). How would the transaction be performed? 7. Show that if longitudinal parity is used as an error detection code, values in a database can still be modified without detection. (Longitudinal parity is computed for the n th bit of each byte; that is, one parity bit is computed and retained for all bits in the 0th position, another parity bit for all bits in the 1st position, etc.) 8. Suppose query Q 1 obtains the median m l of a set S 1 of values, and query Q 2 obtains the median m 2 of a subset S 2 of S 1. If m 1 < m 2, what can be inferred about S 1, S 2, and the elements of S 1 not in S 2? 9. One approach suggested to ensure privacy is the small result rejection, in which the system rejects (returns no result from) any query, the result of which is derived from a small number, for example, five, of records. Show how to obtain sensitive data by using only queries derived from six records. 10. Cite a situation in which the sensitivity of an aggregate is greater than that of its constituent values. Cite a situation in which the sensitivity of an aggregate is less than that of its constituent values.
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