Update Chapter 1 - Solutions.md

Chapter 1 - Solutions.md

@@ -6,6 +6,45 @@ Why would you choose a database system instead of simply storing data in
 operating system files? When would it make sense not to use a database system?
 
 ### `Answer`
+A database is an integrated collection of data, usually so large that it
+has to be stored on secondary storage devices such as disks or tapes. This data can
+be maintained as a collection of operating system files, or stored in a DBMS (database
+management system). The advantages of using a DBMS are:
+- Data independence and efficient access. Database application programs are independent
+of the details of data representation and storage. The conceptual and
+external schemas provide independence from physical storage decisions and logical
+design decisions respectively. In addition, a DBMS provides efficient storage and
+retrieval mechanisms, including support for very large files, index structures and
+query optimization.
+- Reduced application development time. Since the DBMS provides several important
+functions required by applications, such as concurrency control and crash
+recovery, high level query facilities, etc., only application-specific code needs to
+be written. Even this is facilitated by suites of application development tools
+available from vendors for many database management systems.
+- Data integrity and security. The view mechanism and the authorization facilities
+of a DBMS provide a powerful access control mechanism. Further, updates to the
+data that violate the semantics of the data can be detected and rejected by the
+DBMS if users specify the appropriate integrity constraints.
+- Data administration. By providing a common umbrella for a large collection of
+data that is shared by several users, a DBMS facilitates maintenance and data
+administration tasks. A good DBA can effectively shield end-users from the chores
+of fine-tuning the data representation, periodic back-ups etc.
+- Concurrent access and crash recovery. A DBMS supports the notion of a transaction,
+which is conceptually a single user’s sequential program. Users can write
+transactions as if their programs were running in isolation against the database.
+The DBMS executes the actions of transactions in an interleaved fashion to obtain
+good performance, but schedules them in such a way as to ensure that conflicting
+operations are not permitted to proceed concurrently. Further, the DBMS maintains
+a continuous log of the changes to the data, and if there is a system crash,
+it can restore the database to a transaction-consistent state. That is, the actions
+of incomplete transactions are undone, so that the database state reflects only the
+actions of completed transactions. Thus, if each complete transaction, executing
+alone, maintains the consistency criteria, then the database state after recovery
+from a crash is consistent.   
+
+If these advantages are not important for the application at hand, using a collection of
+files may be a better solution because of the increased cost and overhead of purchasing
+and maintaining a DBMS.
 
 ***
 ### Exercise 1.2
@@ -19,6 +58,18 @@ Explain the difference between logical and physical data independence.
 
 ### `Answer`
 
+Logical data independence means that users are shielded from changes
+in the logical structure of the data, while physical data independence insulates users
+from changes in the physical storage of the data. We saw an example of logical data
+independence in the answer to Exercise 1.2. Consider the Students relation from that
+example (and now assume that it is not replaced by the two smaller relations). We
+could choose to store Students tuples in a heap file, with a clustered index on the
+sname field. Alternatively, we could choose to store it with an index on the gpa field,
+or to create indexes on both fields, or to store it as a file sorted by gpa. These storage
+alternatives are not visible to users, except in terms of improved performance, since
+they simply see a relation as a set of tuples. This is what is meant by physical data
+independence
+
 ***
 ### Exercise 1.4
 Explain the difference between external, internal, and conceptual schemas. How are these different schema layers related to the concepts of logical and physical data independence?
@@ -31,6 +82,21 @@ What are the responsibilities of a DBA? If we assume that the DBA is never inter
 
 ### `Answer`
 
+The DBA is responsible for:
+- Designing the logical and physical schemas, as well as widely-used portions of the
+external schema.
+- Security and authorization.
+- Data availability and recovery from failures.
+- Database tuning: The DBA is responsible for evolving the database, in particular
+the conceptual and physical schemas, to ensure adequate performance as user
+requirements change.   
+
+A DBA needs to understand query optimization even if s/he is not interested in running
+his or her own queries because some of these responsibilities (database design
+and tuning) are related to query optimization. Unless the DBA understands the performance
+needs of widely used queries, and how the DBMS will optimize and execute
+these queries, good design and tuning decisions cannot be made.
+
 ***
 ### Exercise 1.6
 
@@ -55,6 +121,20 @@ Which of the following plays an important role in representing information about
 
 ### `Answer`
 
+Let us discuss the choices in turn.
+- The data definition language is important in representing information because it
+is used to describe external and logical schemas.
+- The data manipulation language is used to access and update data; it is not
+important for representing the data. (Of course, the data manipulation language
+must be aware of how data is represented, and reflects this in the constructs that
+it supports.)
+- The buffer manager is not very important for representation because it brings
+arbitrary disk pages into main memory, independent of any data representation.
+- The data model is fundamental to representing information. The data model
+determines what data representation mechanisms are supported by the DBMS.
+- The data definition language is just the specific set of language constructs available
+to describe an actual application’s data in terms of the data model.
+
 ***
 ### Exercise 1.8
 Describe the structure of a DBMS. If your operating system is upgraded to
@@ -79,4 +159,37 @@ and database consistency'?
 
 ### `Answer`
 
+1. A transaction is any one execution of a user program in a DBMS. This is the basic
+unit of change in a DBMS.
+2. A DBMS is typically shared among many users. Transactions from these users
+can be interleaved to improve the execution time of users’ queries. By interleaving
+queries, users do not have to wait for other user’s transactions to complete
+fully before their own transaction begins. Without interleaving, if user A begins
+a transaction that will take 10 seconds to complete, and user B wants to begin
+a transaction, user B would have to wait an additional 10 seconds for user
+A’s transaction to complete before the database would begin processing user B’s
+request.
+3. A user must guarantee that his or her transaction does not corrupt data or insert
+nonsense in the database. For example, in a banking database, a user must guarantee
+that a cash withdraw transaction accurately models the amount a person
+removes from his or her account. A database application would be worthless if
+a person removed 20 dollars from an ATM but the transaction set their balance
+to zero! A DBMS must guarantee that transactions are executed fully and independently
+of other transactions. An essential property of a DBMS is that a
+transaction should execute atomically, or as if it is the only transaction running.
+Also, transactions will either complete fully, or will be aborted and the database
+returned to it’s initial state. This ensures that the database remains consistent.
+4. Strict two-phase locking uses shared and exclusive locks to protect data. A transaction
+must hold all the required locks before executing, and does not release any
+lock until the transaction has completely finished.
+5. The WAL property affects the logging strategy in a DBMS. The WAL, WriteAhead
+Log, property states that each write action must be recorded in the log
+(on disk) before the corresponding change is reflected in the database itself. This
+protects the database from system crashes that happen during a transaction’s
+execution. By recording the change in a log before the change is truly made, the
+database knows to undo the changes to recover from a system crash. Otherwise,
+if the system crashes just after making the change in the database but before
+the database logs the change, then the database would not be able to detect his
+change during crash recovery.
+
 ***