ORACLE隐含列

11G:

Removing Columns from Tables

When you issue an 

ALTER
TABLE...DROP COLUMN

 statement, the column descriptor and the data associated with the target column are removed from each row in the table. You can drop multiple columns with one statement.

The following statements are examples of dropping columns from the 

hr.admin_emp

 table.
The first statement drops only the 

sal

 column:

ALTER TABLE hr.admin_emp DROP COLUMN sal;

The next statement drops both the 

bonus

 and 

comm

 columns:

ALTER TABLE hr.admin_emp DROP (bonus, commission);

Marking Columns Unused

If you are concerned about the length of time it could take to drop column data from all of the rows in a large table, you can use the 

ALTER
TABLE...SET UNUSED

 statement. This statement marks one or more columns as unused, but does not actually remove the target column data or restore the disk space occupied by these columns. However, a column that is marked as unused is not displayed in
queries or data dictionary views, and its name is removed so that a new column can reuse that name. All constraints, indexes, and statistics defined on the column are also removed.

To mark the 

hiredate

 and 

mgr

 columns
as unused, execute the following statement:

ALTER TABLE hr.admin_emp SET UNUSED (hiredate, mgr);

You can later remove columns that are marked as unused by issuing an 

ALTER
TABLE...DROP UNUSED COLUMNS

 statement. Unused columns are also removed from the target table whenever an explicit drop of any particular column or columns of the table is issued.

The data dictionary views 

USER_UNUSED_COL_TABS

, 

ALL_UNUSED_COL_TABS

,
or 

DBA_UNUSED_COL_TABS

 can be used to list all tables
containing unused columns. The 

COUNT

 field shows the number
of unused columns in the table.

SELECT * FROM DBA_UNUSED_COL_TABS;

OWNER                       TABLE_NAME                  COUNT
--------------------------- --------------------------- -----
HR                          ADMIN_EMP                       2

For external tables, the 

SET

 

UNUSED

 statement
is transparently converted into an 

ALTER

 

TABLE

 

DROP

 

COLUMN

 statement.
Because external tables consist of metadata only in the database, the 

DROP

 

COLUMN

 statement
performs equivalently to the 

SET

 

UNUSED

 statement.

Removing Unused Columns

The 

ALTER TABLE...DROP UNUSED COLUMNS

 statement is the
only action allowed on unused columns. It physically removes unused columns from the table and reclaims disk space.

In the 

ALTER TABLE

 statement that follows, the optional
clause 

CHECKPOINT

 is specified. This clause causes a checkpoint
to be applied after processing the specified number of rows, in this case 250. Checkpointing cuts down on the amount of undo logs accumulated during the drop column operation to avoid a potential exhaustion of undo space.

ALTER TABLE hr.admin_emp DROP UNUSED COLUMNS CHECKPOINT 250;