If the PV of the update lies within the PA of a row, only the portion of the row information that is valid during the overlap is updated, while the portions of row that existed before and after the change remain in the table as separate rows. Assume that Fred has to reduce his work for the company for the year of 2005, and agrees to a change in the terms of his contract for that period:
UPDATE employee_vt FOR PORTION OF job_dur FROM DATE'2005-01-01' TO DATE'2006-01-01' SET terms='TW10' WHERE ename='Fred'; SELECT * FROM employee_vt WHERE ename=’Fred’; eid ename term job_start job_end ---- ----- ----- ----------- ----------- 1004 Fred TW10 2005/01/01 2006/01/01 1004 Fred PW12 2001/05/01 2005/01/01 1004 Fred PW12 2006/01/01 9999/12/31
Where there had been one row for Fred in the table before the UPDATE, after the UPDATE there are three rows. The three rows track the changing terms of Fred’s employment, starting with terms PW12, changing to TW10 for one year beginning in 2005, then returning to PW12 in 2006.
Because this was a valid-time UPDATE executed against a valid-time table, all the work of creating new rows and adjusting the valid-times for each row was handled automatically by the database.