CS145 Assignment #4

Due Tuesday, May 6, 1997

Step 4 of Your PDA

This week we shall write and run some SQL queries and data modification commands. It is suggested that you begin to follow the routine of loading your database from your load file, running queries or other commands from a file that contains those SQL statements, and then deleting your data so it doesn't clutter up the database all week and (worse) you don't forget and load the same tuples several times into your relations. Remember that SQL thinks of relations as bags, and so will happily let you insert the same tuple as many times as you ask it to. To clean out a relation R without deleting the schema itself, use command DELETE FROM R;.
  1. (20 pts.) Write five queries on your PDA database, using the select-from-where construct of SQL. To receive full credit, all but perhaps one of your queries should exhibit some interesting feature of SQL: queries over more than one relation, subqueries, aggregation, or grouping, for example. We suggest that you experiment with your SQL commands on a small database (e.g., your hand-created database), before running them on the large database that you loaded in PDA part 3. Initial debugging is much easier when you're operating on small amounts of data. Once you're confident that your queries are working, run them on your complete database. If you discover that most or all of your ``interesting'' queries return an empty answer on your large database, then you probably didn't follow the instructions in Assignment #3 for generating data values that join properly. You will need to modify your data generator accordingly. Turn in a copy of all of your SQL queries, along with a script illustrating their execution. Your script should be sufficient to convince us that your commands run successfully. Please do not, however, turn in query results that are thousands (or hundreds of thousands) of lines long!
  2. (15 pts.) Write five data modification commands on your PDA database. Again, these commands should be ``interesting,'' in the sense that they involve some complex feature, such as inserting the result of a query, updating several tuples at once, or deleting a set of tuples that is more than one but less than all the tuples in a relation. As for part (1), you might want to try out your commands on small data before trying it on your full database. Hand in a script that shows your modification commands running in a convincing fashion.
  3. (15 pts.) In part (1) you probably discovered that some queries run very slowly over your large database. An important technique for improving the performance of queries is to create indexes. An index on an attribute A of relation R allows the database to quickly find all tuples in R with a given value for attribute A. This index is useful if a value of A is specified by your query (in the where-clause). It may also be useful if A is involved in a join that equates it to some other attribute. For example, in the query
         SELECT Bars.address
         FROM Drinkers, Bars
         WHERE   Drinkers.name = 'joe'
             AND Drinkers.frequents = Bars.name;
    we might use an index on Drinkers.name to help us find the tuple for drinker Joe quickly. We might also like an index on Bars.name, so we can take all the bars Joe frequents and quickly find the tuples for those bars to read their addresses.

    In Oracle, you can get an index by the command:

         CREATE INDEX <IndexName> ON <RelName>(<Attribute List>);
    If the attribute list contains more than one attribute, then the index requires values for all the listed attributes to find a tuple. That situation might be helpful if the attributes together form a key, for example. An illustration of the CREATE INDEX command is
         CREATE INDEX DrinkerInd ON Drinkers(name);
         CREATE INDEX BarInd ON Bars(name);
    which creates the two indexes mentioned above. To get rid of an index, you can say DROP INDEX followed by the name of the index. Notice that each index must have a name, even though we only refer to the name if we want to drop the index.

    Create at least two useful indexes for your PDA. Run your queries from part (1) on your large database with the indexes and without the indexes. To time your commands, you may issue the following commands to sqlplus:

    1. TIMING START <TimerName>; starts your timer. Give it whatever name you wish.
    2. TIMING SHOW; prints the current wall-clock time of your current timer.
    3. (There is a way to switch among timers, which is why they are named, but we shall not use this feature.)
    4. TIMING STOP; prints the current time of your timer and stops it.

    Naturally these times may be affected by external factors such as system load, etc. Still, you should see a dramatic difference between the execution times with indexes and the times without. Turn in a script showing your commands to create indexes, and showing the relative times of query execution with and without indexes.

Problem Set

  1. (10 pts.) Exercise 3.7.1(e) [p. 154]. Be sure to explain your reasoning, e.g., what functional dependencies violate the normal forms? What are the dependencies and keys for the relations of the decomposition? If the answer for BCNF and 3NF is different, explain why.

  2. (10 pts.) Exercise 3.7.1(f) [p. 154]. Follow the directions for (1).

  3. (5 pts.) Exercise 4.1.1(b) [p. 187-190].

  4. (5 pts.) Exercise 4.1.1(e) [p. 190].

  5. (10 pts.) Exercise 4.1.1(g) [p. 190].

  6. (10 pts.) Exercise 4.1.1(i) [p. 190].