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Oracle® Database 2 Day + Performance Tuning Guide
11g Release 2 (11.2)

Part Number E10822-03
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2 Oracle Database Performance Method

Performance improvement is an iterative process. Removing the first bottleneck (a point where resource contention is highest) may not lead to performance improvement immediately because another bottleneck might be revealed that has an even greater performance impact on the system. For this reason, the Oracle performance method is iterative. Accurately diagnosing the performance problem is the first step toward ensuring that your changes improve performance.

Typically, performance problems result from a lack of throughput (the amount of work that can be completed in a specified time), unacceptable user or job response time (the time to complete a specified workload), or both. The problem might be localized to specific application modules or span the system.

Before looking at database or operating system statistics, it is crucial to get feedback from the system users and the people paying for the application. This feedback makes it easier to set performance goals. Improved performance can be measured in terms of business goals rather than system statistics.

The Oracle performance method can be applied until performance goals are met or deemed impractical. Because this process is iterative, some investigations may have little impact on system performance. It takes time and experience to accurately pinpoint critical bottlenecks quickly. Automatic Database Diagnostic Monitor (ADDM) implements the Oracle performance method and analyzes statistics to provide automatic diagnosis of major performance problems. Because ADDM can significantly shorten the time required to improve the performance of a system, it is the method used in this guide.

This chapter discusses the Oracle Database performance method and contains the following sections:

Gathering Database Statistics Using the Automatic Workload Repository

Database statistics provide information about the type of load on the database and the internal and external resources used by the database. To accurately diagnose performance problems with the database using ADDM, statistics must be available.

A cumulative statistic is a count such as the number of block reads. Oracle Database generates many types of cumulative statistics for the system, sessions, and individual SQL statements. Oracle Database also tracks cumulative statistics about segments and services. Automatic Workload Repository (AWR) automates database statistics gathering by collecting, processing, and maintaining performance statistics for database problem detection and self-tuning purposes.

By default, the database gathers statistics every hour and creates an AWR snapshot, which is a set of data for a specific time that is used for performance comparisons. The delta values captured by the snapshot represent the changes for each statistic over the time period. Statistics gathered by AWR are queried from memory. The gathered data can be displayed in both reports and views.

The following initialization parameters are relevant for AWR:

The database statistics collected and processed by AWR include:

Time Model Statistics

Time model statistics measure the time spent in the database by operation type. The most important time model statistic is database time (DB time). Database time represents the total time spent in database calls by foreground sessions, and is an indicator of the total instance workload. As shown in Figure 2-1, database time makes up a portion of an application's overall user response time.

Figure 2-1 DB Time in Overall User Response Time

Description of Figure 2-1 follows
Description of "Figure 2-1 DB Time in Overall User Response Time"

A session is a logical entity in the database instance memory that represents the state of a current user login to a database. Database time is calculated by aggregating the CPU time and wait time of all active sessions (sessions that are not idle). For any database request, the CPU time is the sum of the time spent working on the request, while the wait time is the sum of all the waits for various database instance resources. DB time includes only time spent on client processes and does not include time spent on background processes such as PMON.

For example, a user session may involve an online transaction made at an online bookseller consisting of the actions shown in Figure 2-2.

Figure 2-2 DB Time in User Transaction

Description of Figure 2-2 follows
Description of "Figure 2-2 DB Time in User Transaction"

  1. Query for novels by author

    The user performs a search for novels by a particular author. This action causes the application to perform a database query for novels by the author.

  2. Browse results of query

    The user browses the returned list of novels by the author and accesses additional details, such as user reviews and inventory status. This action causes the application to perform additional database queries.

  3. Add item to cart

    After browsing details about the novels, the user decides to add one novel to the shopping cart. This action causes the application to make a database call to update the shopping cart.

  4. Checkout

    The user completes the transaction by checking out, using the address and payment information previously saved at the bookseller's Web site from a previous purchase. This action causes the application to perform various database operations to retrieve the user's information, add a new order, update the inventory, and generate an e-mail confirmation.

For each of the preceding actions, the user makes a request to the database, as represented by the down arrow in Figure 2-2. The CPU time spent by the database processing the request and the wait time spent waiting for the database are considered DB time, as represented by the shaded areas. After the request is completed, the results are returned to the user, as represented by the up arrow. The space between the up and down arrows represents the total user response time for processing the request, which contains other components besides DB time, as illustrated in Figure 2-1.


DB time is measured cumulatively from when the instance started. Because DB time combines times from all non-idle user sessions, DB time can exceed the time elapsed since the instance started. For example, an instance that has run 5 minutes could have four active sessions whose cumulative DB time is 20 minutes.

The objective of database tuning is to reduce database time. In this way, you can improve the overall response time of user transactions in the application.

Wait Event Statistics

Wait events are incremented by a session to indicate that the session had to wait for an event to complete before being able to continue processing. When a session has to wait while processing a user request, the database records the wait by using one of a set of predefined wait events. The events are then grouped into wait classes, such as User I/O and Network. Wait event data reveals symptoms of problems that might be affecting performance, such as latch, buffer, or I/O contention.

Session and System Statistics

A large number of cumulative database statistics are available on a system and session level. Some of these statistics are collected by AWR.

Active Session History Statistics

The Active Session History (ASH) statistics are samples of session activity in the database. The database samples active sessions every second and stores them in a circular buffer in the System Global Area (SGA). Any session that is connected to the database and using CPU, or is waiting for an event that does not belong to the idle wait class, is considered an active session. By capturing only active sessions, a manageable set of data is represented. The size of the data is directly related to the work being performed, rather than the number of sessions allowed on the database.

Using the DB time example described in "Time Model Statistics", samples of session activity are collected from the online transaction made at the bookseller's Web site, represented as vertical lines below the horizontal arrow in Figure 2-3.

Figure 2-3 Active Session History

Description of Figure 2-3 follows
Description of "Figure 2-3 Active Session History"

The light vertical lines represent samples of inactive session activity that are not captured in the ASH statistics. The bold vertical lines represent samples of active sessions that are captured at:

  • 7:38, while novels by the author are being queried

  • 7:42, while the user is browsing the query results

  • 7:50, when one novel is added to the shopping cart

  • 7:52, during the checkout process

Table 2-1 lists ASH statistics collected for the active sessions, along with examples of the session ID (SID), module, SQL ID, session state, and wait events that are sampled.

Table 2-1 Active Session History

Time SID Module SQL ID State Event



Book by author



db file sequential read



Get review ID






Add item to cart



buffer busy wait






log file sync

High-Load SQL Statistics

SQL statements that are consuming the most resources produce the highest load on the system, based on criteria such as elapsed time and CPU time.

Using the Oracle Performance Method

Performance tuning using the Oracle performance method is driven by identifying and eliminating bottlenecks in the database, and by developing efficient SQL statements. Database tuning is performed in two phases: proactively and reactively.

In the proactive tuning phase, you must perform tuning tasks as part of your daily database maintenance routine, such as reviewing ADDM analysis and findings, monitoring the real-time performance of the database, and responding to alerts.

In the reactive tuning phase, you must respond to issues reported by users, such as performance problems that may occur for only a short duration of time, or performance degradation to the database over a period of time.

SQL tuning is an iterative process to identify, tune, and improve the efficiency of high-load SQL statements.

Applying the Oracle performance method involves the following:

To improve database performance, you must apply these principles iteratively.

Preparing the Database for Tuning

This section lists and describes the steps that must be performed before the database can be properly tuned.

To prepare the database for tuning

  1. Get feedback from users.

    Determine the scope of the performance project and subsequent performance goals, and determine performance goals for the future. This process is key for future capacity planning.

  2. Check the operating systems of all systems involved with user performance.

    Check for hardware or operating system resources that are fully utilized. List any overused resources for possible later analysis. In addition, ensure that all hardware is functioning properly.

  3. Ensure that the STATISTICS_LEVEL initialization parameter is set to TYPICAL (default) or ALL to enable the automatic performance tuning features of Oracle Database, including AWR and ADDM.

  4. Ensure that the CONTROL_MANAGEMENT_PACK_ACCESS initialization parameter is set to DIAGNOSTIC+TUNING (default) or DIAGNOSTIC to enable ADDM.

Tuning the Database Proactively

This section lists and describes the proactive steps required to keep the database properly tuned on a regular basis. Perform these steps as part of your daily maintenance of Oracle Database. Repeat the tuning process until your performance goals are met or become impossible to achieve because of other constraints.

To tune the database proactively

  1. Review the ADDM findings, as described in Chapter 3, "Automatic Database Performance Monitoring".

    ADDM automatically detects and reports on performance problems with the database, including most of the "Common Performance Problems Found in Oracle Databases". The results are displayed as ADDM findings on the Database Home page in Oracle Enterprise Manager (Enterprise Manager). Reviewing these findings enables you to quickly identify the performance problems that require your attention.

  2. Implement the ADDM recommendations, as described in Chapter 3, "Automatic Database Performance Monitoring".

    With each ADDM finding, ADDM automatically provides a list of recommendations for reducing the impact of the performance problem. Implementing a recommendation applies the suggested changes to improve the database performance.

  3. Monitor performance problems with the database in real time, as described in Chapter 4, "Monitoring Real-Time Database Performance".

    The Performance page in Enterprise Manager enables you to identify and respond to real-time performance problems. By drilling down to the appropriate pages, you can identify and resolve performance problems with the database in real time, without having to wait until the next ADDM analysis.

  4. Respond to performance-related alerts, as described in Chapter 5, "Monitoring Performance Alerts".

    The Database Home page in Enterprise Manager displays performance-related alerts generated by the database. Typically, resolving the problems indicated by these alerts improves database performance.

  5. Validate that any changes made have produced the desired effect, and verify that the users experience performance improvements.

Tuning the Database Reactively

This section lists and describes the steps required to tune the database based on user feedback. This tuning procedure is considered reactive. Perform this procedure periodically when performance problems are reported by the users.

To tune the database reactively

  1. Run ADDM manually to diagnose current and historical database performance when performance problems are reported by the users, as described in Chapter 6, "Manual Database Performance Monitoring".

    In this way you can analyze current database performance before the next ADDM analysis, or analyze historical database performance when you were not proactively monitoring the system.

  2. Resolve transient performance problems, as described in Chapter 7, "Resolving Transient Performance Problems".

    The Active Session History (ASH) reports enable you to analyze transient performance problems with the database that are short-lived and do not appear in the ADDM analysis.

  3. Resolve performance degradation over time, as described in Chapter 8, "Resolving Performance Degradation Over Time".

    The Automatic Workload Repository (AWR) Compare Periods report enables you to compare database performance between two periods of time, and resolve performance degradation that may happen from one time period to another.

  4. Validate that the changes made have produced the desired effect, and verify that the users experience performance improvements.

  5. Repeat these steps until your performance goals are met or become impossible to achieve due to other constraints.

Tuning SQL Statements

This section lists and describes the steps required to identify, tune, and optimize high-load SQL statements.

To tune SQL statements

  1. Identify high-load SQL statements, as described in Chapter 9, "Identifying High-Load SQL Statements".

    Use the ADDM findings and the Top SQL section to identify high-load SQL statements that are causing the greatest contention.

  2. Tune high-load SQL statements, as described in Chapter 10, "Tuning SQL Statements".

    You can improve the efficiency of high-load SQL statements by tuning them using SQL Tuning Advisor.

  3. Optimize data access paths, as described in Chapter 11, "Optimizing Data Access Paths".

    You can optimize the performance of data access paths by creating the proper set of materialized views, materialized view logs, and indexes for a given workload by using SQL Access Advisor.

  4. Analyze the SQL performance impact of SQL tuning and other system changes by using SQL Performance Analyzer.

    To learn how to use SQL Performance Analyzer, see Oracle Database Real Application Testing User's Guide.

  5. Repeat these steps until all high-load SQL statements are tuned for greatest efficiency.

Common Performance Problems Found in Oracle Databases

This section lists and describes common performance problems found in Oracle databases. By following the Oracle performance method, you should be able to avoid these problems. If you experience these problems, then repeat the steps in the Oracle performance method, as described in "Using the Oracle Performance Method", or consult the appropriate section that addresses these problems: