Physically Removing the Column
The other method for dropping an unnecessary table column is through the use of the the alter table name drop column colname statement. This statement actually removes all data from the column and eliminates the column from the table definition. This operation may take more time to complete than marking the column as unused, because Oracle has to go through all blocks of the table and actually remove the column data in order to reclaim the space used by that column. The following is an example of physically removing a table column:
ALTER TABLE d_table DROP COLUMN d_column CASCADE CONSTRAINTS;
