Optimizer Statistics

 The optimizer statistics are a collection of data that describe details about the database and the objects in the database. The statistics provide a statistically correct picture of data storage and distribution usable by the optimizer when evaluating access paths.

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Optimizer statistics include the following:

• Table statistics

  These include the number of rows, number of blocks, and average row length.
  

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• Column statistics

  These include the number of distinct values and nulls in a column and the distribution of data.

• Index statistics

  These include the number of leaf blocks and index levels.

• System statistics

  These include CPU and I/O performance and utilization.
  

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Oracle Database gathers optimizer statistics on all database objects automatically and maintains these statistics as an automated maintenance task. You can also gather statistics manually using the DBMS_STATS package. This PL/SQL package can modify, view, export, import, and delete statistics.

Note

Optimizer statistics are created for the purposes of query optimization and are stored in the data dictionary. Do not confuse these statistics with performance statistics visible through dynamic performance views.

Optimizer Statistics Advisor is built-in diagnostic software that analyzes how you are currently gathering statistics, the effectiveness of existing statistics gathering jobs, and the quality of the gathered statistics. Optimizer Statistics Advisor maintains rules, which embody Oracle best practices based on the current feature set. In this way, the advisor always provides the most up-to-date recommendations for statistics gathering.

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