Altering Tables

Reasons for Using the ALTER TABLE Statement

You can use the ALTER TABLE statement to perform any of the following actions that affect a table:

• Modify physical characteristics (INITRANS or storage parameters)

• Move the table to a new segment or tablespace

• Explicitly allocate an extent or deallocate unused space

• Add, drop, or rename columns, or modify an existing column definition (data type, length, default value, NOT NULL integrity constraint, column expression (for virtual columns), and encryption properties.)

• Modify the logging attributes of the table

• Modify the CACHE/NOCACHE attributes

• Add, modify or drop integrity constraints associated with the table

• Enable or disable integrity constraints or triggers associated with the table

• Modify the degree of parallelism for the table

• Rename a table

• Put a table in read-only mode and return it to read/write mode

• Add or modify index-organized table characteristics

• Alter the characteristics of an external table

• Add or modify LOB columns

• Add or modify object type, nested table, or varray columns

Many of these operations are discussed in succeeding sections.

Altering Physical Attributes of a Table

When altering the transaction entry setting INITRANS of a table, note that a new setting for INITRANS applies only to data blocks subsequently allocated for the table.

The storage parameters INITIAL and MINEXTENTS cannot be altered. All new settings for the other storage parameters (for example, NEXT, PCTINCREASE) affect only extents subsequently allocated for the table. The size of the next extent allocated is determined by the current values of NEXT and PCTINCREASE, and is not based on previous values of these parameters.

Moving a Table to a New Segment or Tablespace

The ALTER TABLE...MOVE statement enables you to relocate data of a nonpartitioned table or of a partition of a partitioned table into a new segment, and optionally into a different tablespace for which you have quota. This statement also lets you modify any of the storage attributes of the table or partition, including those which cannot be modified using ALTER TABLE. You can also use the ALTER TABLE...MOVE statement with a COMPRESS clause to store the new segment using table compression.

One important reason to move a table to a new tablespace (with a new datafile) is to eliminate the possibility that old versions of column data—versions left on now unused portions of the disk due to segment shrink, reorganization, or previous table moves—could be viewed by bypassing the access controls of the database (for example with an operating system utility). This is especially important with columns that you intend to modify by adding transparent data encryption.

The following statement moves the hr.admin_emp table to a new segment, specifying new storage parameters:

ALTER TABLE hr.admin_emp MOVE
      STORAGE ( INITIAL 20K
                NEXT 40K
                MINEXTENTS 2
                MAXEXTENTS 20
                PCTINCREASE 0 );

Moving a table changes the rowids of the rows in the table. This causes indexes on the table to be marked UNUSABLE, and DML accessing the table using these indexes will receive an ORA-01502 error. The indexes on the table must be dropped or rebuilt. Likewise, any statistics for the table become invalid and new statistics should be collected after moving the table.

If the table includes LOB column(s), this statement can be used to move the table along with LOB data and LOB index segments (associated with this table) which the user explicitly specifies. If not specified, the default is to not move the LOB data and LOB index segments.

Manually Allocating Storage for a Table

Oracle Database dynamically allocates additional extents for the data segment of a table, as required. However, perhaps you want to allocate an additional extent for a table explicitly. For example, in an Oracle Real Application Clusters environment, an extent of a table can be allocated explicitly for a specific instance.

A new extent can be allocated for a table using the ALTER TABLE...ALLOCATE EXTENT clause.

You can also explicitly deallocate unused space using the DEALLOCATE UNUSED clause of ALTER TABLE. This is described in "Reclaiming Wasted Space".

Modifying an Existing Column Definition

Use the ALTER TABLE...MODIFY statement to modify an existing column definition. You can modify column data type, default value, column constraint, column expression (for virtual columns) and column encryption.

You can increase the length of an existing column, or decrease it, if all existing data satisfies the new length. You can change a column from byte semantics to CHAR semantics or vice versa. You must set the initialization parameter BLANK_TRIMMING=TRUE to decrease the length of a non-empty CHAR column.

If you are modifying a table to increase the length of a column of data type CHAR, realize that this can be a time consuming operation and can require substantial additional storage, especially if the table contains many rows. This is because the CHAR value in each row must be blank-padded to satisfy the new column length.

Adding Table Columns

To add a column to an existing table, use the ALTER TABLE...ADD statement.

The following statement alters the hr.admin_emp table to add a new column named bonus:

ALTER TABLE hr.admin_emp
      ADD (bonus NUMBER (7,2));

If a new column is added to a table, the column is initially NULL unless you specify the DEFAULT clause. When you specify a default value, the database immediately updates each row with the default value. Note that this can take some time, and that during the update, there is an exclusive DML lock on the table. For some types of tables (for example, tables without LOB columns), if you specify both a NOT NULL constraint and a default value, the database can optimize the column add operation and greatly reduce the amount of time that the table is locked for DML.

You can add a column with a NOT NULL constraint only if the table does not contain any rows, or you specify a default value.

Renaming Table Columns

Oracle Database lets you rename existing columns in a table. Use the RENAME COLUMN clause of the ALTER TABLE statement to rename a column. The new name must not conflict with the name of any existing column in the table. No other clauses are allowed with the RENAME COLUMN clause.

The following statement renames the comm column of the hr.admin_emp table.

ALTER TABLE hr.admin_emp
      RENAME COLUMN comm TO commission;

As noted earlier, altering a table column can invalidate dependent objects. However, when you rename a column, the database updates associated data dictionary tables to ensure that function-based indexes and check constraints remain valid.

Oracle Database also lets you rename column constraints. This is discussed in "Renaming Constraints".

Dropping Table Columns

You can drop columns that are no longer needed from a table, including an index-organized table. This provides a convenient means to free space in a database, and avoids your having to export/import data then re-create indexes and constraints.

You cannot drop all columns from a table, nor can you drop columns from a table owned by SYS. Any attempt to do so results in an error.

ALTER TABLE hr.admin_emp DROP COLUMN sal;

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

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

SELECT * FROM DBA_UNUSED_COL_TABS;

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

ALTER TABLE hr.admin_emp DROP UNUSED COLUMNS CHECKPOINT 250;

Placing a Table in Read-Only Mode

You can place a table in read-only mode with the ALTER TABLE...READ ONLY statement, and return it to read/write mode with the ALTER TABLE...READ WRITE statement. An example of a table for which read-only mode makes sense is a configuration table. If your application contains configuration tables that are not modified after installation and that must not be modified by users, your application installation scripts can place these tables in read-only mode.

To place a table in read-only mode, you must have the ALTER TABLE privilege on the table or the ALTER ANY TABLE privilege. In addition, the COMPATIBILE initialization parameter must be set to 11.1.0 or greater.

The following example places the SALES table in read-only mode:

ALTER TABLE SALES READ ONLY;

The following example returns the table to read/write mode:

ALTER TABLE SALES READ WRITE;

When a table is in read-only mode, operations that attempt to modify table data are disallowed. The following operations are not permitted on a read-only table:

• All DML operations on the table or any of its partitions

• TRUNCATE TABLE

• SELECT FOR UPDATE

• ALTER TABLE ADD/MODIFY/RENAME/DROP COLUMN

• ALTER TABLE SET COLUMN UNUSED

• ALTER TABLE DROP/TRUNCATE/EXCHANGE (SUB)PARTITION

• ALTER TABLE UPGRADE INCLUDING DATA or ALTER TYPE CASCADE INCLUDING TABLE DATA for a type with read-only table dependents

• Online redefinition

• FLASHBACK TABLE

The following operations are permitted on a read-only table:

• SELECT

• CREATE/ALTER/DROP INDEX

• ALTER TABLE ADD/MODIFY/DROP/ENABLE/DISABLE CONSTRAINT

• ALTER TABLE for physical property changes

• ALTER TABLE DROP UNUSED COLUMNS

• ALTER TABLE ADD/COALESCE/MERGE/MODIFY/MOVE/RENAME/SPLIT (SUB)PARTITION

• ALTER TABLE MOVE

• ALTER TABLE ENABLE ROW MOVEMENT and ALTER TABLE SHRINK

• RENAME TABLE and ALTER TABLE RENAME TO

• DROP TABLE

• ALTER TABLE DEALLOCATE UNUSED

• ALTER TABLE ADD/DROP SUPPLEMENTAL LOG