Beginning Oracle Database 11g Administration From Novice to Professional

CHAPTER 17 SQL TUNING 385
much time and attention to individual statements. The amount of effort we are willing to
expend is usually governed by business requirements and the return on our investment.
The most frequently cited cause of inefficient SQL is the failure of the query optimizer
to generate an efficient query execution plan, but there are many others. Here are
some examples:
equally efficient. In Chapter 2, you saw ten solutions to the problem of listing the
status of all suppliers who supply hammers. Failure to use advanced features of
the language is a common cause of inefficient SQL.
SQL. For example, giving users the ability to generate new types of queries (a.k.a.
ad hoc queries) is usually a perfect recipe for poor performance. Another example
of poor application development practices is insufficient testing.
Examples are inadequate indexing and partitioning of data, and insufficient attention
to disk layouts.
time passes. A perfect example is the absence of an archiving strategy to keep the
amount of data under control.
It bears repeating that hardware limitations—including CPU speed, memory size,
and disk speed—play a big role in performance. If the system does not have enough
memory, data may have to be frequently retrieved from disk. The workload handled by
the database server also plays a major role in performance; your SQL statement will run
slowly if it has to compete for resources with SQL statements submitted by other users.
Often this is the result of poor coordination of workloads. For example, it is poor practice
to perform batch processing during the day when most OLTP work is performed. Sometimes
this is the result of poor capacity planning. For example, an online store may not
have properly planned for the increase in transaction volumes during popular holidays.
Ways to Improve SQL
Given enough time, effort, and money, it is usually possible to extract more performance
out of any SQL statement. However, it is not usually possible to give so much time and
attention to individual statements. The amount of effort we are willing to expend is usually
governed by business requirements and the return on our investment. The goal
is usually to bring performance of poorly performing SQL statements to a level that is
acceptable to the users of the database. The following sections describe some techniques
that can be used to improve performance.