CS145 Assignment #4

Due in class, Tuesday, Nov. 9, 2004

Step 4 of Your PDA

This week we shall write and run some SQL queries and do some other operations. We suggest you institute a 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 its schema, use command DELETE FROM R;.

Please use the on-line submit procedure, as described in Homework 3 to turn in your PDA.

Also, please go to http://www-db.stanford.edu/~ullman/fcdb/oracle.html and look over the topics for which we provide information. We're not expecting you to memorize the list, but it pays to look at the titles once, so if you ever encounter a problem there is some chance your memory will be jogged, and you'll go look at this document again. Similarly, please peruse the tables of contents for the currently relevant documents: or-intro.html, or-faq.html, or-nulls.html, or-time.html, and or-nonstandard.html . Again, the idea is that if you've seen it once, you will probably remember its existence if you need it. Especially, the document on ways that Oracle differs from the SQL standard (or-nonstandard) will save a lot of aggrievation when you encounter things like "Oracle won't allow the word EXCEPT."

  1. Write three queries on your PDA database, using the select-from-where construct of SQL. To receive full credit, each of your queries should exhibit some interesting feature of SQL: subqueries, or queries over more than one relation, 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 some or all of your queries return an empty answer on your large database, check whether you followed the instructions in PDA #3 for generating data values that join properly. You may need to modify your data generator accordingly. Turn in a copy 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. Write three data modification commands on your PDA database. 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 the queries in (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. Create two views on top of your database schema. Show your CREATE VIEW statements and the response of the system. Also, show a query involving each view and the system response (but truncate the response if there are more than a few tuples produced). Finally, show a script of what happens when you try to modify your view, say by inserting a new tuple into it. Are either of your views updatable? Tell why or why not? (Updatable views are discussed in Section 6.7.4 of the text. Essentially, a view is updatable if it is a selection on one base table.)

  4. Add two attribute-based and two tuple-based CHECK constraints to relations of your database schema. Remember that these constraints are more limited in Oracle than in the SQL definition; see The Guide to Nonstandard Oracle Features for details. Show the revised schema, its successful declaration, and the response of Oracle to inserts or updates that violate the constraints.

  5. 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 find quickly 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 = 'fred'
         AND Drinkers.frequents = Bars.name;
    we might use an index on Drinkers.name to help us find the tuple for drinker Fred quickly. We might also like an index on Bars.name, so we can take all the bars Fred 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>)
         TABLESPACE INDX;
    Note:

    If the attribute list for an index 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)
         TABLESPACE INDX;
     CREATE INDEX BarInd ON Bars(name)
         TABLESPACE INDX;
    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.

    In order to get Oracle to use its indexes, you often need to tell it to use the better of the two query optimizers that it has available, the so-called "cost-based optimizer." Before taking any measurements of query running time, you must say to sqlplus: 

         ALTER SESSION SET OPTIMIZER_MODE = ALL_ROWS;

    Now, you are ready to run your experiments. 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. (There is a way to switch among timers, which is why they are named, but we shall not use this feature.)

    3. 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.

    Note: Often, students discover that indexes appear to slow down the execution of queries. There are two issues you should consider:

    1. A query involving several relations will not speed up unless indexes to support all of the selections and equijoins are available. A single index may only increase the number of disk I/O's needed (to get index blocks), without affecting the query as a whole.

    2. The second time you run a query may take much less time than the first, because the second time data is cached in main memory. Data may be cached in the main memory of the machine running Oracle. If many students are using Oracle at the same time, the Oracle machine's cache will probably drop your data if you wait a few seconds. We suggest that you perform only one timing experiment in a session; at least exit from sqlplus and start it again.