DB2 Recovery Expert for Multiplatforms

Chapter 5. Log analysis 77

update sales set region='South' where region='Ontario-South';

The obvious way to undo this would be:

update sales set region='Ontario-South' where region='South';

However, undoing changes this way can introduce data integrity problems. What

if there were already rows in the sales table that were region South before the

original update was done. Undoing the update as suggested above would also

affect those rows, which is not what we want.

Therefore, Recovery Expert uses a different approach. Log analysis works

row-by-row. The original statement actually changed 13 rows in the SALES table,

so the Undo SQL produced (Example B-4 on page 236) uses 13 statements to

undo the changes caused by one SQL statement (shown in bold ...