Monday, November 2, 2009

Automatic Statistics Gathering in Oracle10g

Oracle10g has one of the automated feature which is called Automatic Statistics gathering. Oracle gathers statistics on all database objects automatically and maintains those statistics in a regularly scheduled maintenance job. When the DB is created, job is automatically created and started to collect statistics on database object with missing or stale statistics. The job GATHER_STATS_JOB calls the procedure dbms_stats.gather_database_stats_job_proc to gather statistics during the maintenance window.

What does GATHER_DATABASE_STATS_JOB_PROC do? Unfortunately, the package body is wrapped, so without access to the original source code, the implementation details are obscured. This procedure collects statistics on database objects when the object has no previously gathered statistics or the existing statistics are stale because the underlying object has been modified significantly (more than 10% of the rows).

This is an internal procedure, but its operates in a very similar fashion to the DBMS_STATS.GATHER_DATABASE_STATS procedure using the GATHER AUTO option. The primary difference is that the DBMS_STATS.GATHER_DATABASE_STATS_JOB_PROC procedure prioritizes the database objects that require statistics, so that those objects which most need updated statistics are processed first. This ensures that the most-needed statistics are gathered before the maintenance window closes.

How do we enable this feature? Automatic statistics gathering is enabled by default when a database is created, or when a database is upgraded from an earlier database release. The following condition should be met to enable this feature.

1. GATHER_STATS_JOB should be enabled.
2. statistics_level should be TYPICAL or ALL.

We can enable the job as below when the job is already disabled for any reason.

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

SQL> connect sys/password@orcl as sysdba
3 END;
4 /

PL/SQL procedure successfully completed.


What time does automatic statistic collection run? By default this job runs within a maintenance window between 10 P.M to 6 A.M week nights and all day on weekends. But in oracle11g, i believe, the maintenance window is not same as Oracle10g.

Do we need to collect statistics in other then the scheduled window? In few Scenario's, it would be required to gather statistics manually.

1. In some cases, we might need to collect the statistics in specific objects other than this maintenance window. If the table data are changed rapidly and optimizer behaves differently due to data changes, then we need to gather statistics manually. We can not wait till the maintenance window open.

2. If the table is bigger and it is failing in the maintenance window for some reason, then you can lock that table during the maintenance window and unlock the table and gather the statistics manually and lock back again.

3. If you have specific table and you want to gather statistics with different option, then you can lock those tables during the maintenance window and gather statistics manually during day time.

Should we change the parameter values in Automatic statistic collection? Oracle recommends not to change the default values for automatic statistics collection. In case, if you need to gather statistics something differently, then lock the table during the maintenance window and unlock the table and gather statistics manually with specfic parameter and lock the table back again.

We have get_param function to find out the parameter values. Also set_param procedure to change the default values for dbms_stats pacakge parameters.

No comments: