Computer Science 143 Homework 3

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Part 1: Go Long or Go Wide?

The website keyvalues.com allows startups and some large companies to discuss their company culture,
something that some of you will find to be much more important than a huge paycheck. The purpose of
this site is to provide candidates and other interested parties an in depth look at the company culture on a
variety of dimensions. The content tends to discuss cultural values that are often overlooked in interviews.

The table below shows some of them:

Team Values Personal Health Daily Routines Engineering Career Growth
Engages with Community Work/Life Balance Eats Lunch Together Open Source Contributor Promotes from Within
Team is Diverse Ideal for Parents Light Meetings Start-to-Finish Ownership Good for Junior Devs
Risk Taking > Stability Fosters Psychological Safety Thoughtful Office Layout Fast-Paced High Retention
Strategy Company Properties
Engineering-Driven Remote Work OK
Data-Driven
Rapidly Growing Team

Exercise

Help your professor! (Though I’ve already done this) Take a look at the data. Write a query that creates a
0/1 matrix from this data. The rows should correspond to companies, the columns should correspond to cultural
values. The value of each cell should be a 0 or 1 – a 1 if the value is associated with the company, 0 otherwise.
The query may be tedious, but in the “lots of copy/paste” way. Some of you may find a much better way to do
this that does not require that. I do not know if it is possible, so if you are adventurous, you may want to research
it.
To grade this problem, please submit your query, the number of rows in the output, and the
number of columns in the output. The output itself you can submit as a text file if you wish, but
it is probably too tedious to read.
With this 0/1 matrix, we can then create visualizations of which companies are similar to others, and which values
are similar to others using singular value decomposition, principal component analysis, or multiple correspondence
analysis. We won’t do that for this class, but if you are interested, have some fun with the data.
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Part 2: EXPLAIN Yourself

In the solutions for Homework 2, I gave several ways of writing the same query. For part 1B:
1. No Subquery with DISTINCT
2. SELECT within a SELECT
3. JOIN as a Filter
4. Using an IN Subquery as a Filter
5. Formal Left Semijoin

Run a SQL EXPLAIN on each of these queries. Just copy and paste the query from the solutions into
a text editor and make any corrections so that it will run. The EXPLAIN output varies by implementation, but for MySQL, you can read more about it here: https://dev.mysql.com/doc/refman/8.0/en/
explain-output.html.

Provide the output of each SQL EXPLAIN. Then write a paragraph (approximately) detailing
what you notice overall (do not write a paragraph for each one). Which query appears to be
the most efficient (intentionally vague)? What is your definition of efficient? Pay attention
to the number of rows, the filtered column, the extra column, and any information provided
about joins.
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Part 3: Join and Optimization Algorithms

Chapter 12 is rather tedious, so I have asked everyone to read this chapter. It is so tedious that it would
take multiple lectures for me to address the class and also answer questions. While we have not focused
on nitty-gritty math involved in disk block reads (aside from on the exam), it is important to be aware of
it as you may be asked in which situation to use which join algorithm, and you will probably want to use
this math to justify your response. Remember, data wins arguments.

Exercises

1. Suppose we have two relations L and R. The nested-loop join algorithm is presented below:
If L has 100,000 tuples, how many times is relation R scanned?
2. The indexed nested-join loop is similar, but instead of doing a linear scan over R, we build an index
on it.
(a) How many times is each tuple in both L and R scanned?
(b) In the worst case, what is the number of block transfers required for this join if we assume R is
indexed with a B+-tree?
3. In terms of disk I/O, which of the three join algorithms is the most efficient: naive nested-join loop,
block nested-join loop, indexed nested-join loop and why? The answer probably depends on several
factors. Describe these factors and come up with a heuristic describing when you may want to use
each.

4. Prove that the following equivalencies hold. Explain how you can apply them to improve the efficiency
of certain queries. Feel free to define relations if you would like:
E1 ./θ (E2 − E3) = (E1 ./θ E2 − E1 ./θ E3)
5. For the pair of expressions, give instances of relations that show the expressions are not equivalent.
ΠA (R − S) and ΠA(R) − ΠA(S)
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