SQL Plan Management - Creating SQL plan baselines

Introduction

Do you ever experience performance regressions because an execution plan has changed for the worse? If you have, then we have an elegant solution for you in 11g called SQL Plan Management (SPM). The next four posts on our blog will cover SPM in detail. Let's begin by reviewing the primary causes for plan changes. 

Execution plan changes occur due to various system changes. For example, you might have (manually or automatically) updated statistics for some objects, or changed a few optimizer-related parameters. A more dramatic change is a database upgrade (say from 10gR2 to 11g). All of these changes have the potential to cause new execution plans to be generated for many of your SQL statements. Most new plans are obviously improvements because they are tailored to the new system environment, but some might be worse leading to performance regressions. It is the latter that cause sleepless nights for many DBAs. 

DBAs have several options for addressing these regressions. However, what most DBAs want is simple: plans should only change when they will result in performance gains. In other words, the optimizer should not pick bad plans, period. 

This first post in our series, describes the concepts of SQL Plan Management and how to create SQL plan baselines. The second part will describe how and when these SQL plan baselines are used. The third part will discuss evolution, the process of adding new and improved plans to SQL plan baselines. Finally, the fourth part will describe user interfaces and interactions with other Oracle objects (like stored outlines).

SQL Plan Management

SQL Plan Management (SPM) allows database users to maintain stable yet optimal performance for a set of SQL statements. SPM incorporates the positive attributes of plan adaptability and plan stability, while simultaneously avoiding their shortcomings. It has two main objectives:

1. prevent performance regressions in the face of database system changes
2. offer performance improvements by gracefully adapting to database system changes

A managed SQL statement is one for which SPM has been enabled. SPM can be configured to work automatically or it can be manually controlled either wholly or partially (described later). SPM helps prevent performance regressions by enabling the detection of plan changes for managed SQL statements. For this purpose, SPM maintains, on disk, a plan history consisting of different execution plans generated for each managed SQL statement. An enhanced version of the Oracle optimizer, called SPM aware optimizer, accesses, uses, and manages this information which is stored in a repository called the SQL Management Base (SMB). 

The plan history enables the SPM aware optimizer to determine whether the best-cost plan it has produced using the cost-based method is a brand new plan or not. A brand new plan represents a plan change that has potential to cause performance regression. For this reason, the SPM aware optimizer does not choose a brand new best-cost plan. Instead, it chooses from a set of accepted plans. An accepted plan is one that has been either verified to not cause performance regression or designated to have good performance. A set of accepted plans is called a SQL plan baseline, which represents a subset of the plan history. 

A brand new plan is added to the plan history as a non-accepted plan. Later, an SPM utility verifies its performance, and keeps it as a non-accepted plan if it will cause a performance regression, or changes it to an accepted plan if it will provide a performance improvement. The plan performance verification process ensures both plan stability and plan adaptability. 

Credit goes to the below website(s):

https://blogs.oracle.com/optimizer/entry/sql_plan_management_part_1_of_4_creating_sql_plan_baselines

http://www.oracle.com/technetwork/database/bi-datawarehousing/twp-sql-plan-management-11gr2-133099.pdf

http://www.databasejournal.com/features/oracle/article.php/3896411/Understanding-SQL-Plan-Baselines-in-Oracle-Database-11g.htm