Oracle performance tuning is a big subject and whole books have been written on it which means we can't possibly cover the subject in a few web pages. What we can do though is zoom in on the most important aspects and provide some tips and techniques as well as where to find more information.
So let's get started.
What is Oracle Performance Tuning?This might seem like an obvious question. "It's tuning the Oracle database so that it's faster, isn't it?"
That is one answer but
is not really the best answer. Speed implies response time and this may not always be the factor that needs tuning. It may be overall throughput or other resources that need tuning, such as memory usage, cpu, i/o in general or reads specifically or writes specifically.
The answer in each specific case also needs to include whether it's the Oracle database per se that needs to be tuned or whether it's the application using it. There may be a fine line between the two but it defintely exists.
It's better, therefore, to generalise the definition as reducing resource usage by the database.
Now that we've defined it, where do we start?
The answer to that is you start where the problem is. In other words you need to stop thinking of tuning the system as a whole and start considering individual subsystems and application modules.
You need to start by gathering information about the performance problem. Does it occur at a particular time of day or day of the week or at month end or just at the end of the financial/calendar year? Does it affect a group of users or just one in particular or is it system wide? If it affects just one group of users you need to find out what they are doing differently to everyone else. The more information you have before you start, the more effective you will be at solving the problem.
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The tools available to help you diagnose and resolve performance issues depend to a certain extent on the version of the database you are using. Oracle 10g and upwards have more tools available than earlier versions. In general the tools available include:
When faced with an SQL statement that you suspect could be improved, before changing it you need to know what it's current execution plan is. This can be achieved by preceding the statement with the explain plan clause and then running it. This writes the predicted execution plan that the statement would use if it were to be run into the plan table from where it can be retrieved by calling DBMS_XPLAN.DISPLAY.
As the predicted execution plan may not be entirely accurate you can determine the actual execution plan of the statement after it has been run by using DBMS_XPLAN.DISPLAY_CURSOR.
Often this is the only tool required for Oracle performance tuning as most problems are due to SQL statements that are not using the optimal path to access the data.
SQL trace is an Oracle database facility that provides individual performance information about SQL statements either at the instance or session level.
The information gathered about SQL statements comprises:
If the statement's cursor is closed, row source information is also provided including:
TKPROF is a program that is run against Oracle trace files to format them and generate a readable output. It can also:
display the actual execution plan of a statement create a SQL script that stores statistics in the database
The DBMS_STATS package provides a number of routines for generating and manipulating statistics on tables and indexes in the database. These statistics are then used by the Oracle cost-based optimiser when determining the optimal execution plan for SQL statements.
This package enables you to manage SQL execution plans to ensure that the plan and therefore the performance of the corresponding SQL statements are unchanged irrespective of any changes to the system such as more data, a new version of the Oracle database, or new applications.
This package enables the management of stored outlines (stored data for an an execution plan for a specific SQL statement) in the Oracle database. It enables the
Oracle optimizer to re-create an execution plan equivalent to the original plan. In future releases of Oracle this will be deprecated in favour of SQL plan management (DBMS_SPM).
Introduced with Oracle 10g, AWR helps with Oracle performance tuning by collecting, processing and maintaining performance information which can be displayed in reports and/or views.
This analyzes the data collected by AWR to identify any performance issues and provides recommendations to correct problems.
More information on these tools can be found in the Oracle Database Performance Tuning Guide available from the Oracle Technology Network.
More detail is available in the following tutorials on:
Tuning index usage and full table scans in your SQL