Beginning Oracle Database 11g Administration From Novice to Professional

CHAPTER 17
SQL Tuning
Every limbo boy and girl
All around the limbo world
Gonna do the limbo rock
All around the limbo clock
—“Limbo Rock,” recorded by Chubby Checker in 1962
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Perhaps the most complex problem in database administration is SQL tuning, and it
may not be a coincidence that I left it for the very end. The paucity of books devoted to
SQL tuning is perhaps further evidence of the difficulty of the topic.
The only way to interact with Oracle, to retrieve data, to change data, to administer
the database, is SQL. Oracle itself uses SQL to perform all the work that it does behind the
scenes. SQL performance is therefore the key to database performance; all database performance
problems are really SQL performance problems even if they express themselves
as contention for resources.
In this chapter, I will present some of the causes of inefficient SQL and some of the
common techniques of making SQL more efficient, but we will spend most of our time
working through a case study. I will present a fairly typical SQL statement and improve it in
stages until it hits the theoretical maximum level of performance that is possible to achieve.
Defining Efficiency
The efficiency of an SQL statement is measured by the amount of computing resources
such as CPU cycles used in producing the output. Reducing the consumption of resources
is the goal of SQL tuning. Elapsed time is not a good measure of the efficiency of an SQL
statement because it is not always proportional to the amount of resources consumed.
For example, contention for CPU cycles and disk I/O causes execution delays. The number
of logical read operations is a better way to measure the consumption of computing
resources because it is directly proportional to the amount of resources consumed—the
fewer the number of logical read operations, the less CPU consumption.
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