Wednesday, October 7, 2009

Refreshing Stale Statistics

Oracle optimizer use the statistics information to choose the right path and execute the query efficiently. It is important to maintain the recent statistics to run the reports efficiently. Oracle highly recommends to use DBMS_STATS to gather statistics. Why oracle recommends to use DBMS_STATS? Click here to answer your question. This article is based on Oracle10g.

DBMS_STATS package has wonderful feature that capable of analyzing the stale statistics. I am going to discuss about collecting stale statistics in dbms_stats package.

In general, Gathering statistics consumes lot of resource and CPU time. Once we gathered statistics on a table, we do not need to collect the statistics on the same table until we make reasonable amount of data changes. Let us say, we have a schema called sales. This schema has lot of tables and many table has huge number of records. We schedule to analyze the entire schema every day at 2AM. In day to day DML activities, some of the tables are not having any changes or very minimum changes. In this scenario, we do not need to analyze the tables which are having very minimum changes or no changes. But scheduler automatically start analyzing all the tables in the schema at 2AM every day. This process unnecessarily consuming extra resource and degrade the server performance.

How do we stop analyzing the tables when there is no DML activity or very minimal DML activity? Yes... We can... Oracle introduced feature in DBMS_STATS package where oracle collect statistics on schema level or database level, only when the statistics are stale or out of date.

What is stale statistics? Oracle will record an approximate count of the number of rows that have been inserted,updated, and deleted in a table. The information will be recorded in user_tab_modifications view. When that count reaches a threshold percentage of the number of rows in the table , then the statistics are considered stale. The table monitoring should be enabled for recording the DML changes in user_tab_modification view. In oracle10g, Oracle automatically enable table monitoring and record the DML changes in user_tab_modifications view. Prior to oracle10g, we need to enable the table monitoring manually.

How do we enable table monitoring? In oracle10g, the table monitoring is default when statistic_level parameter is TYPICAL. Prior to Oracle10g, we need to enable table monitoring manually. Prior to Oracle10g, the below command is used to enable or disable the table monitoring. But since Oracle10g, the below command does not have any effect.


What is threshold percentage? Oracle automatically determines the threshold. Oracle doesn't officially document the threshold, so the threshold, and the entire algorithm, is subject to change over time.

Let me give an example how to analyze stale statistics :

Oracle Database 10g Enterprise Edition Release - Production
With the Partitioning, OLAP and Data Mining options

3 ownname => 'SALES',
4 estimate_percent => 20,
5 block_sample => TRUE,
6 method_opt => 'FOR COLUMNS SIZE 10',
7 options => 'GATHER AUTO',
8 cascade => TRUE);
9 END;
10 /

PL/SQL procedure successfully completed.


Note : It is very important that, we should use GATHER AUTO or GATHER STALE to analyze the stale statistics. Also table monitoring is mandatory. Since oracle10g, the table monitoring is enabled by default. So we do not need to worry about table monitoring since oracle10g.

This feature is very useful for large and complex databases where refreshing statistics for all objects can cause a heavy drain on server resources.

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