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1. a In an environment in which several users share access to a single database can one user ever block

1. (a)
In an environment in which several users share access to a single
database, can one user ever block another’s 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 nth
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 Q1 obtains the median ml of a set S1 of values, and query Q2 obtains the median m2 of a subset S2 of S1. If m1 < m2, what can be inferred about S1, S2, and the elements of S1 not in S2?
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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