ALTER COLUMN

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ALTER COLUMN is a subcommand of ALTER TABLE. You can use ALTER COLUMN to do the following:

Set, change, or drop a column's DEFAULT constraint.
Set or drop a column's NOT NULL constraint.
New in v21.2: Set, change, or drop an ON UPDATE expression.
Change a column's data type.

Note:

The ALTER COLUMN statement performs a schema change. For more information about how online schema changes work in CockroachDB, see Online Schema Changes.

Note:

Support for altering column types is experimental, with certain limitations. For details, see Altering column data types.

Tip:

This command can be combined with other ALTER TABLE commands in a single statement. For a list of commands that can be combined, see ALTER TABLE. For a demonstration, see Add and rename columns atomically.

Synopsis

Required privileges

The user must have the CREATE privilege on the table.

Parameters

Parameter	Description
`table_name`	The name of the table with the column to modify.
`column_name`	The name of the column to modify.
`SET DEFAULT a_expr`	The new default value.
`typename`	The new data type you want to use. Support for altering column types is experimental, with certain limitations. For details, see Altering column data types.
`USING a_expr`	How to compute a new column value from the old column value.

View schema changes

This schema change statement is registered as a job. You can view long-running jobs with SHOW JOBS.

Altering column data types

Support for altering column data types is experimental, with certain limitations. To enable column type altering, set the enable_experimental_alter_column_type_general session variable to true.

The following are equivalent in CockroachDB:

ALTER TABLE ... ALTER ... TYPE
ALTER TABLE ... ALTER COLUMN TYPE
ALTER TABLE ... ALTER COLUMN SET DATA TYPE

For examples of ALTER COLUMN TYPE, Examples.

Limitations on altering data types

You cannot alter the data type of a column if:

The column is part of an index.
The column has CHECK constraints.
The column owns a sequence.
The ALTER COLUMN TYPE statement is part of a combined ALTER TABLE statement.
The ALTER COLUMN TYPE statement is inside an explicit transaction.

Note:

Most ALTER COLUMN TYPE changes are finalized asynchronously. Schema changes on the table with the altered column may be restricted, and writes to the altered column may be rejected until the schema change is finalized.

Examples

Set or change a `DEFAULT` value

Setting the DEFAULT value constraint inserts the value when data's written to the table without explicitly defining the value for the column. If the column already has a DEFAULT value set, you can use this statement to change it.

The following example inserts the Boolean value true whenever you inserted data to the subscriptions table without defining a value for the newsletter column.

> ALTER TABLE subscriptions ALTER COLUMN newsletter SET DEFAULT true;

Remove `DEFAULT` constraint

If the column has a defined DEFAULT value, you can remove the constraint, which means the column will no longer insert a value by default if one is not explicitly defined for the column.

> ALTER TABLE subscriptions ALTER COLUMN newsletter DROP DEFAULT;

Set `NOT NULL` constraint

Setting the NOT NULL constraint specifies that the column cannot contain NULL values.

> ALTER TABLE subscriptions ALTER COLUMN newsletter SET NOT NULL;

Remove `NOT NULL` constraint

If the column has the NOT NULL constraint applied to it, you can remove the constraint, which means the column becomes optional and can have NULL values written into it.

> ALTER TABLE subscriptions ALTER COLUMN newsletter DROP NOT NULL;

Convert a computed column into a regular column

You can convert a stored, computed column into a regular column by using ALTER TABLE.

In this example, create a simple table with a computed column:

> CREATE TABLE office_dogs (
    id INT PRIMARY KEY,
    first_name STRING,
    last_name STRING,
    full_name STRING AS (CONCAT(first_name, ' ', last_name)) STORED
  );

Then, insert a few rows of data:

> INSERT INTO office_dogs (id, first_name, last_name) VALUES
    (1, 'Petee', 'Hirata'),
    (2, 'Carl', 'Kimball'),
    (3, 'Ernie', 'Narayan');

> SELECT * FROM office_dogs;

+----+------------+-----------+---------------+
| id | first_name | last_name |   full_name   |
+----+------------+-----------+---------------+
|  1 | Petee      | Hirata    | Petee Hirata  |
|  2 | Carl       | Kimball   | Carl Kimball  |
|  3 | Ernie      | Narayan   | Ernie Narayan |
+----+------------+-----------+---------------+
(3 rows)

The full_name column is computed from the first_name and last_name columns without the need to define a view. You can view the column details with the SHOW COLUMNS statement:

> SHOW COLUMNS FROM office_dogs;

+-------------+-----------+-------------+----------------+------------------------------------+-------------+
| column_name | data_type | is_nullable | column_default |       generation_expression        |   indices   |
+-------------+-----------+-------------+----------------+------------------------------------+-------------+
| id          | INT       |    false    | NULL           |                                    | {"primary"} |
| first_name  | STRING    |    true     | NULL           |                                    | {}          |
| last_name   | STRING    |    true     | NULL           |                                    | {}          |
| full_name   | STRING    |    true     | NULL           | concat(first_name, ' ', last_name) | {}          |
+-------------+-----------+-------------+----------------+------------------------------------+-------------+
(4 rows)

Now, convert the computed column (full_name) to a regular column:

> ALTER TABLE office_dogs ALTER COLUMN full_name DROP STORED;

Check that the computed column was converted:

> SHOW COLUMNS FROM office_dogs;

+-------------+-----------+-------------+----------------+-----------------------+-------------+
| column_name | data_type | is_nullable | column_default | generation_expression |   indices   |
+-------------+-----------+-------------+----------------+-----------------------+-------------+
| id          | INT       |    false    | NULL           |                       | {"primary"} |
| first_name  | STRING    |    true     | NULL           |                       | {}          |
| last_name   | STRING    |    true     | NULL           |                       | {}          |
| full_name   | STRING    |    true     | NULL           |                       | {}          |
+-------------+-----------+-------------+----------------+-----------------------+-------------+
(4 rows)

The computed column is now a regular column and can be updated as such:

> INSERT INTO office_dogs (id, first_name, last_name, full_name) VALUES (4, 'Lola', 'McDog', 'This is not computed');

> SELECT * FROM office_dogs;

+----+------------+-----------+----------------------+
| id | first_name | last_name |      full_name       |
+----+------------+-----------+----------------------+
|  1 | Petee      | Hirata    | Petee Hirata         |
|  2 | Carl       | Kimball   | Carl Kimball         |
|  3 | Ernie      | Narayan   | Ernie Narayan        |
|  4 | Lola       | McDog     | This is not computed |
+----+------------+-----------+----------------------+
(4 rows)

Alter the formula for a computed column

To alter the formula for a computed column, you must DROP and ADD the column back with the new definition. Take the following table for instance:

> CREATE TABLE x (
a INT NULL,
b INT NULL AS (a * 2) STORED,
c INT NULL AS (a + 4) STORED,
FAMILY "primary" (a, b, rowid, c)
);

CREATE TABLE


Time: 4ms total (execution 4ms / network 0ms)

Add a computed column d:

> ALTER TABLE x ADD COLUMN d INT AS (a // 2) STORED;

ALTER TABLE


Time: 199ms total (execution 199ms / network 0ms)

If you try to alter it, you'll get an error:

> ALTER TABLE x ALTER COLUMN d INT AS (a // 3) STORED;

invalid syntax: statement ignored: at or near "int": syntax error
SQLSTATE: 42601
DETAIL: source SQL:
ALTER TABLE x ALTER COLUMN d INT AS (a // 3) STORED
                             ^
HINT: try \h ALTER TABLE

However, you can drop it and then add it with the new definition:

> SET sql_safe_updates = false;
> ALTER TABLE x DROP COLUMN d;
> ALTER TABLE x ADD COLUMN d INT AS (a // 3) STORED;
> SET sql_safe_updates = true;

SET


Time: 1ms total (execution 0ms / network 0ms)

ALTER TABLE


Time: 195ms total (execution 195ms / network 0ms)

ALTER TABLE


Time: 186ms total (execution 185ms / network 0ms)

SET


Time: 0ms total (execution 0ms / network 0ms)

Convert to a different data type

The TPC-C database has a customer table with a column c_credit_lim of type DECIMAL(10,2):

> SELECT column_name, data_type FROM [SHOW COLUMNS FROM customer] WHERE column_name='c_credit_lim';

  column_name  |   data_type
---------------+----------------
  c_credit_lim | DECIMAL(10,2)
(1 row)

To change the data type from DECIMAL to STRING:

Set the enable_experimental_alter_column_type_general session variable to true:
```
> SET enable_experimental_alter_column_type_general = true;
```

Alter the column type:

> ALTER TABLE customer ALTER c_credit_lim TYPE STRING;

NOTICE: ALTER COLUMN TYPE changes are finalized asynchronously; further schema changes on this table may be restricted until the job completes; some writes to the altered column may be rejected until the schema change is finalized

Verify the type:

> SELECT column_name, data_type FROM [SHOW COLUMNS FROM customer] WHERE column_name='c_credit_lim';

  column_name  | data_type
---------------+------------
  c_credit_lim | STRING
(1 row)

Change a column type's precision

The TPC-C customer table contains a column c_balance of type DECIMAL(12,2):

> SELECT column_name, data_type FROM [SHOW COLUMNS FROM customer] WHERE column_name='c_balance';

  column_name |   data_type
--------------+----------------
  c_balance   | DECIMAL(12,2)
(1 row)

To increase the precision of the c_balance column from DECIMAL(12,2) to DECIMAL(14,2):

> ALTER TABLE customer ALTER c_balance TYPE DECIMAL(14,2);

> SELECT column_name, data_type FROM [SHOW COLUMNS FROM customer] WHERE column_name='c_balance';

  column_name |   data_type
--------------+----------------
  c_balance   | DECIMAL(14,2)
(1 row)

Change a column's type using an expression

You can change the data type of a column and create a new, computed value from the old column values, with a USING clause. For example:

> SELECT column_name, data_type FROM [SHOW COLUMNS FROM customer] WHERE column_name='c_discount';

  column_name |  data_type
--------------+---------------
  c_discount  | DECIMAL(4,4)
(1 row)

> SELECT c_discount FROM customer LIMIT 10;

  c_discount
--------------
      0.1569
      0.4629
      0.2932
      0.0518
      0.3922
      0.1106
      0.0622
      0.4916
      0.3072
      0.0316
(10 rows)

> ALTER TABLE customer ALTER c_discount TYPE STRING USING ((c_discount*100)::DECIMAL(4,2)::STRING || ' percent');

NOTICE: ALTER COLUMN TYPE changes are finalized asynchronously; further schema changes on this table may be restricted until the job completes; some writes to the altered column may be rejected until the schema change is finalized

> SELECT column_name, data_type FROM [SHOW COLUMNS FROM customer] WHERE column_name='c_discount';

  column_name | data_type
--------------+------------
  c_discount  | STRING
(1 row)

> SELECT c_discount FROM customer LIMIT 10;

   c_discount
-----------------
  15.69 percent
  46.29 percent
  29.32 percent
  5.18 percent
  39.22 percent
  11.06 percent
  6.22 percent
  49.16 percent
  30.72 percent
  3.16 percent
(10 rows)

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ALTER COLUMN

Synopsis

Required privileges

Parameters

View schema changes

Altering column data types

Limitations on altering data types

Examples

Set or change a `DEFAULT` value

Remove `DEFAULT` constraint

Set `NOT NULL` constraint

Remove `NOT NULL` constraint

Convert a computed column into a regular column

Alter the formula for a computed column

Convert to a different data type

Change a column type's precision

Change a column's type using an expression

See also

ALTER COLUMN

Synopsis

Required privileges

Parameters

View schema changes

Altering column data types

Limitations on altering data types

Examples

Set or change a DEFAULT value

Remove DEFAULT constraint

Set NOT NULL constraint

Remove NOT NULL constraint

Convert a computed column into a regular column

Alter the formula for a computed column

Convert to a different data type

Change a column type's precision

Change a column's type using an expression

See also

Set or change a `DEFAULT` value

Remove `DEFAULT` constraint

Set `NOT NULL` constraint

Remove `NOT NULL` constraint