The fact that the application is being developed
at an accelerated pace does not imply that the deliverables will be any quicker to generate.
Chapter 5: Developing and Implementing Applications 159
Iterative Column Definitions
During the development process, your column definitions may change frequently. You can
drop columns from existing tables. You can drop a column immediately, or you can mark it as
UNUSED to be dropped at a later time. If the column is dropped immediately, the action may
impact performance. If the column is marked as unused, there will be no impact on performance.
The column can actually be dropped at a later time when the database is less heavily used.
To drop a column, use either the set unused clause or the drop clause of the alter table
command. You cannot drop a pseudocolumn, a column of a nested table, or a partition key column.
In the following example, column Col2 is dropped from a table named TABLE1:
alter table TABLE1 drop column Col2;
You can mark a column as unused, as shown here:
alter table TABLE1 set unused column Col3;
Marking a column as unused does not release the space previously used by the column. You
can also drop any unused columns:
alter table TABLE1 drop unused columns;
You can query USER_UNUSED_COL_TABS, DBA_UNUSED_COL, and ALL_UNUSED_COL_TABS
to see all tables with columns marked as unused.
NOTE
Once you have marked a column as unused, you cannot access that
column.
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