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Oracle Technology Network
Cost Control: Inside the Oracle Optimizer

By Donald K. Burleson
OTN Member since 2001

Designing new applications for the Oracle Cost-Based Optimizer? Here's the latest information about how it works.

This article has the following sections:


In Oracle we now see 11g extended optimizer statistics, an alternative to dynamic_sampling for estimating result set sizes.

PART 2 - CBO Statistics

The most important key to success with the CBO is to carefully define and manage your statistics. In order for the CBO to make an intelligent decision about the best execution plan for your SQL, it must have information about the table and indexes that participate in the query. When the CBO knows the size of the tables and the distribution, cardinality, and selectivity of column values, the CBO can make an informed decision and almost always generates the best execution plan.

As a review, the CBO gathers information from many sources, and he has the lofty goal of using DBA-provided metadata to always make the "best" execution plan decision:

Oracle uses data from many sources to make an execution plan

Let's examine the following areas of CBO statistics and see how to gather top-quality statistics for the CBO and how to create an appropriate CBO environment for your database.

Getting top-quality statistics for the CBO. The choices of executions plans made by the CBO are only as good as the statistics available to it. The old-fashioned analyze table and dbms_utility methods for generating CBO statistics are obsolete and somewhat dangerous to SQL performance. As we may know, the CBO uses object statistics to choose the best execution plan for all SQL statements.

The dbms_stats utility does a far better job in estimating statistics, especially for large partitioned tables, and the better statistics result in faster SQL execution plans. Here is a sample execution of dbms_stats with the OPTIONS clause:

exec dbms_stats.gather_schema_stats( - 
  ownname          => 'SCOTT', - 
  options          => 'GATHER AUTO', - 
  estimate_percent => dbms_stats.auto_sample_size, - 
  method_opt       => 'for all columns size repeat', - 
  degree           => 34 - 
Here is another dbms_stats example that creates histograms on all indexes columns:

There are several values for the OPTIONS parameter that we need to know about:

  • GATHER_ reanalyzes the whole schema
  • GATHER EMPTY_ only analyzes tables that have no existing statistics
  • GATHER STALE_ only reanalyzes tables with more than 10 percent modifications (inserts, updates,   deletes)
  • GATHER AUTO_ will reanalyze objects that currently have no statistics and objects with stale statistics.  Using GATHER AUTO is like combining GATHER STALE and GATHER EMPTY.

Note that both GATHER STALE and GATHER AUTO require monitoring. If you issue the ALTER TABLE XXX MONITORING command, Oracle tracks changed tables with the dba_tab_modifications view. Below we see that the exact number of inserts, updates and deletes are tracked since the last analysis of statistics:

SQL> desc dba_tab_modifications;

 Name                Type
 TABLE_NAME          VARCHAR2(30)
 INSERTS             NUMBER
 UPDATES             NUMBER
 DELETES             NUMBER
 TRUNCATED           VARCHAR2(3)

The most interesting of these options is the GATHER STALE option. Because all statistics will become stale   quickly in a robust OLTP database, we must remember the rule for GATHER STALE is > 10% row change   (based on num_rows at statistics collection time). Hence, almost every table except read-only tables will be reanalyzed with the GATHER STALE option, making the GATHER STALE option best for systems that are       largely read-only. For example, if only five percent of the database tables get significant updates, then only        five percent of the tables will be reanalyzed with the GATHER STALE option.

Automating sample size with dbms_stats.The better the quality of the statistics, the better the job that the    CBO will do when determining your execution plans. Unfortunately, doing a complete analysis on a large  database could take days, and most shops must sample your database to get CBO statistics. The goal is to take a large enough sample of the database to provide top-quality data for the CBO.

Now that we see how the dbms_stats option works, let's see how to specify an adequate sample size for dbms_stats.

In earlier releases, the DBA had to guess what percentage of the database provided the best sample size and sometimes underanalyzed the schema. Starting with Oracle9i Database, the estimate_percent argument is a great way to allow Oracle's dbms_stats to automatically estimate the "best" percentage of a segment to sample when gathering statistics:

estimate_percent => dbms_stats.auto_sample_size

After collecting automatic sample sizes, you can verify the accuracy of the automatic statistics sampling by    looking at the sample_size column on any of these data dictionary views:


Note that Oracle generally chooses a sample_size from 5 to 20 percent when using automatic sampling, depending on the size of the tables and the distribution of column values. Remember, the better the quality of  your statistics, the better the decision of the CBO.


In Oracle we now see 11g extended optimizer statistics, an alternative to dynamic_sampling for estimating result set sizes.

Now that we understand the value of CBO statistics, let's look at ways that the CBO statistics are managed in a successful Oracle shop.

The Ion for Oracle tool is the easiest way to analyze Oracle performance and Ion allows you to spot hidden performance trends.




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