23 Managing Automatic System Tasks Using the Maintenance Window

Oracle Database is preconfigured to perform some routine database maintenance tasks so that you can run them at times when the system load is expected to be light. You can specify for such a time period a resource plan that controls the resource consumption of those maintenance tasks. When the designated time period ends, the database can switch to a different resource plan that lowers the resource allocation for any remaining maintenance tasks.

This chapter consists of the following sections:

Maintenance Windows
Automatic Statistics Collection Job
Resource Management

Maintenance Windows

The Oracle Scheduler enables you to create time windows during which jobs are automatically run. A typical Scheduler window defines a start time, a duration, and optionally a resource plan to activate. A Scheduler job can then name a window as its schedule. (When the window "opens," the job begins to run.) In addition, windows can be combined into window groups, and if a job names a window group as its schedule instead of naming a window, the job runs whenever any of the windows in the window group opens.

Two Scheduler windows are predefined upon installation of Oracle Database:

WEEKNIGHT_WINDOW starts at 10 p.m. and ends at 6 a.m. every Monday through Friday.
WEEKEND_WINDOW covers whole days Saturday and Sunday.

Together these windows constitute the MAINTENANCE_WINDOW_GROUP in which all system maintenance tasks are scheduled. Oracle Database uses the maintenance windows for automatic statistics collection and for some other internal system maintenance jobs. If you are using the Resource Manager, you can also assign resource plans to these windows.

You can adjust the predefined maintenance windows to a time suitable to your database environment using the DBMS_SCHEDULER.SET_ATTRIBUTE procedure. For example, the following script moves the WEEKNIGHT_WINDOW to midnight to 8 a.m. every weekday morning:

EXECUTE DBMS_SCHEDULER.SET_ATTRIBUTE(
   'WEEKNIGHT_WINDOW', 
   'repeat_interval',
   'freq=daily;byday=MON, TUE, WED, THU, FRI;byhour=0;byminute=0;bysecond=0');

You can also use the SET_ATTRIBUTE procedure to adjust any other property of a window. For example, the following script sets resource plan DEFAULT_MAINTENANCE_PLAN for the WEEKNIGHT_WINDOW:

EXECUTE DBMS_SCHEDULER.SET_ATTRIBUTE (
   'WEEKNIGHT_WINDOW',
   'resource_plan',
   'DEFAULT_MAINTENANCE_PLAN');

In this case, if you have already enabled a different resource plan, Oracle Database will make the DEFAULT_MAINTENANCE_PLAN active when the WEEKNIGHT_WINDOW opens, and will reactivate the original resource plan when the WEEKNIGHT_WINDOW closes.

Automatic Statistics Collection Job

A Scheduler program GATHER_STATS_PROG and Scheduler job GATHER_STATS_JOB are predefined on installation of Oracle Database. GATHER_STATS_PROG collects optimizer statistics for all objects in the database for which there are no statistics or only stale statistics. GATHER_STATS_JOB is defined on GATHER_STATS_PROG and is scheduled to run in the MAINTENANCE_WINDOW_GROUP.

If you prefer to manage statistics collection manually, you can disable the job as follows:

EXECUTE DBMS_SCHEDULER.DISABLE('GATHER_STATS_JOB');

Resource Management

A Resource Manager consumer group, AUTO_TASK_CONSUMER_GROUP, is predefined on installation of Oracle Database, and a Scheduler job class AUTO_TASKS_JOB_CLASS is defined based on this consumer group. The GATHER_STATS_JOB is defined to run in the AUTO_TASKS_JOB_CLASS job class.

When a resource plan is activated in the system, GATHER_STATS_JOB and any internal system tasks conform to the resource consumption specified for AUTO_TASK_CONSUMER_GROUP in this resource plan.