23 Managing Automatic System Tasks Using the Maintenance Window

Maintenance Windows

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, for space management, and for some other internal system maintenance jobs.

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');

Note that the duration of the window was already eight hours, so the script did not need to change it.

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.

Predefined Automatic System Tasks

The following jobs are among the automatic system tasks that are predefined to run in the maintenance windows:

• Automatic Statistics Collection Job

• Automatic Segment Advisor Job

EXECUTE DBMS_SCHEDULER.DISABLE('GATHER_STATS_JOB');

Resource Management for Automatic System Tasks

If you want to control how CPU resources are allocated to the automatic system tasks so that these tasks do not significantly impact the performance of other database applications, you can create a resource plan and assign the plan to the maintenance windows. (These windows initially have no resource plan assigned.) This resource plan must include directives for the predefined consumer group AUTO_TASK_CONSUMER_GROUP, which is the Resource Manager consumer group under which the automatic system tasks run.

The connection between the automatic system tasks and the AUTO_TASK_CONSUMER_GROUP is made as follows: A Scheduler job class AUTO_TASKS_JOB_CLASS is predefined and names AUTO_TASK_CONSUMER_GROUP as its resource_consumer_group attribute. The automatic system tasks GATHER_STATS_JOB and AUTO_SPACE_ADVISOR_JOB are defined to run in the AUTO_TASKS_JOB_CLASS job class.