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I am not sure why you think logging the supplemental information into
the undo results in a double penalty. Every block change in Oracle
creates a before image (undo), and an after-image (redo). Supplemental
information always pertain to the before-image (since the columns are
not going to change), and is only logged in the before-image part
(undo).
Having said that, supplemental logging does add an overhead, especially
for tables that do not have any unique/primary constraints defined.
Note if constraint maintenance overhead is the reason that you do not
want to define such a constraint, you can always define RELY
constraints to direct the supplemental logging mechanism to log a
subset of columns that uniquely identify the row, although there is no
unique/primary constraints being maintained on the table.
Regards
-Joydip
Received on Fri May 05 2006 - 08:52:51 CDT