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SQL Tuning as a Phase of Oracle Tuning

Oracle Tips by Burleson Consulting

The tuning of SQL is a very integral component of Oracle tuning, and one that can have an immediate impact on performance. However, it is very tempting to dive into SQL tuning without performing the prerequisite tuning activities.

There is a very specific order to Oracle tuning. All Oracle database are organized as a hierarchy, with global components affecting subcomponents, as you can see in Figure 1-5.

Figure 5: The Oracle tuning hierarchy

This hierarchy is very important because any bad tuning at a high level will affect all activities at the level beneath it. Here are the details about the Oracle tuning hierarchy:

  • Environmental tuning   Environmental tuning involves the tuning of the  database server, the network, and the disk I/O subsystem. Only after these components are tuned can the Oracle Remote DBA begin tuning the Oracle database.
  • Database server tuning   The tuning of the Oracle database  server is a prerequisite for all Oracle tuning. If the database server experiences shortages of RAM or CPU, no amount of Oracle tuning will remedy the problem.
  • Network tuning   The network substrate must be tuned to ensure that there are no packet shipping issues at the network protocol level.  For details on tuning Oracle network communications, see Oracle High-performance Tuning with STATSPACK, Chapter 7.
  • Disk tuning   The tuning of the disk I/O subsystem is an  absolute prerequisite to Oracle tuning. Disk I/O bottlenecks and disk access problems must be addressed before Oracle tuning begins.
  • Instance tuning   Once the external environment is tuned, the next phase is the tuning of the Oracle instance. This involves tuning the SGA memory region and tuning the behavior of the Oracle background processes. With regards to SQL tuning, this is the step where the default optimizer_mode for all SQL in the database is set.  For details about Oracles optimizer_mode parameter, see Chapters 14 and 15.
  • Object tuning   Once the instance is tuned, each Oracle object must be tuned for optimal performance. This phase involves getting the proper setting for all storage parameters, especially those storage parameters that affect I/O. The settings for pctfree, pctused, and freelists all have an important impact on SQL performance.
  • SQL tuning   Once all of the general tuning has been accomplished, you are ready to embark on SQL tuning. This phase involves the identification of high-use SQL statements, tuning the statements, and ensuring that the optimal execution plan is made permanent.

Database Design and SQL Performance

Note that we have deliberately left out one of the most important factors in SQL speed, the design of the original data structures, outlined in Figure 1-6. The amount of data normalization and the amount of planned redundancy within the Oracle tables will have a dramatic impact on the speed of the queries. As you will learn later in this text, by adding redundant columns to tables (denormalization), you can avoid expensive SQL joins and improve performance.

Figure 6: The relative performance benefits of different tuning activities

We have omitted this discussion because it is rarely possible for the Oracle Remote DBA to change the table design once the application has gone into a production environment. However, we will be discussing column replication techniques in a later chapter, to show you how you can avoid a SQL table join by replicating a data column from one table to another (as shown in Figure 1-7), and how you can provide a trigger to keep the replicated data item up to date.

Figure 7: Improved performance with column replication

Next, let’s look at the challenges that are faced by the Oracle Remote DBA when undertaking SQL tuning.

This is an excerpt from "Oracle High-Performance SQL Tuning" by Donald K. Burleson, published by Oracle Press.

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