Dropping Columns
Oracle 8i introduced the ability to drop a column from a table. Prior to this it was necessary to drop the entire table and rebuild it. Now you can mark a column as unused (logical delete) or delete it completely (physical delete). Logical Delete On large tables the process of physically removing a column can be very time and resource consuming. For this reason you may decide to logically delete it. ALTER TABLE table_name SET UNUSED (column_name); ALTER TABLE table_name SET UNUSED (column_name1, column_name2); Once this is done the columns will no longer be visible to the user. If at a later date you have time to physically delete the columns this can be done using the following. ALTER TABLE table_name DROP UNUSED COLUMNS; On large tables you can reduce the amount of undo logs accumulated by using the CHECKPOINT option which forces a checkpoint after the specified number of rows have been processed. ALTER TABLE table_name DROP UNUSED COLUMNS CHECKPOINT 250;