Query Optimization Overview
In this class we'll start a week's focus on query optimization. There
are two readings:
To successfully complete this reading, be ready to write a good
response and be prepared for a good in class discussion, I suggest the
following strategy:
- Read the overview first. It will help give you an idea of what to
focus on when reading Selinger et al (especially since it has a
section on Selinger et al.) For the overview:
- For those of you who don't know what a group-by query is, a
group-by query is a query where you divide tuples into groups and
apply aggregate operators to each group. For example, you might look
through a relation about sailors and their ages, and count the
number of sailors who were each age.
- A "block" in a query is approximately equal to one
select from where block. A "nested query" is one that
consists of more than one block.
- For a basic definition of semi-join, check out
the
Wikipedia
article.
It's worth noting that this term has also been co-opted by a slightly
different one in the case of distributed databases, which is the
definition given in some books. You don't really need to know what it
means, but now you have a reference if you're curious.
- For Selinger et al:
- Don't worry about the details of the cost formulas, just get the
main idea
- Don't worry about the details of the join algorithms; we'll study
those a bit more when we study query execution methods. The
important thing to know is that there are several ways of performing a
join, and that some of them leave tuples sorted, and others leave the
tuples unsorted.
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Rachel Pottinger
E-mail Address: ![rap [at] cs [dot] ubc [dot] ca](/~rap/address.jpg)
Office Location: CICSR 345
Phone: (604)822-0436
Fax:(604)822-5485
Postal/Courier address:
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University of British Columbia
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