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Oracle Job Scheduling

Chapter 3 - Time-Based Job Scheduling

Complex Date Rules for Job Execution

It would appear that all of the above expressions give exactly the same run schedule regardless of which syntax is used.  In practice this not true because PL/SQL expressions can allow the run schedules of jobs to slide.

The scheduler attempts to execute all jobs on time, but in practice there is often a small delay.  When a job is executed, the first thing that happens is the next run date is calculated using the specified repeat interval.  Since most PL/SQL expressions use either the sysdate or systimestamp functions, the actual start date may be slightly later than the jobs original next run date.  Over several iterations of the job, this could add up to a noticeable difference between the times the job is expected to run and when it actually does run.  Table 3.12 gives an example of the sort of slide that might be seen if the scheduler is consistently 10 seconds late in executing a job that was originally intended to run a 09:00:00 each day.

RUN

ACTUAL START DATE

NEXT RUN DATE

1

01-JAN-2004 09:00:10

02-JAN-2004 09:00:10

2

02-JAN-2004 09:00:20

03-JAN-2004 09:00:20

3

03-JAN-2004 09:00:30

04-JAN-2004 09:00:30

4

04-JAN-2004 09:00:40

05-JAN-2004 09:00:40

5

05-JAN-2004 09:00:50

06-JAN-2004 09:00:50

6

06-JAN-2004 09:01:00

07-JAN-2004 09:01:00

Table 3.12 – The time slide phenomenon


The above text is an excerpt from:


Oracle Job Scheduling
Creating Robust Task Management with dbms_job and Oracle10g dbms_scheduler

ISBN 0-9744486-6-4

by Dr. Timothy S. Hall
 

http://www.rampant-books.com/book_2005_1_scheduling.htm


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