The DROP COLUMN statement is part of ALTER TABLE and removes columns from a table.
The DROP COLUMN statement performs a schema change. For more information about how online schema changes work in CockroachDB, see Online Schema Changes.
When used in an explicit transaction combined with other schema changes to the same table, DROP COLUMN can result in data loss if one of the other schema changes fails or is canceled. To work around this, move the DROP COLUMN statement to its own explicit transaction or run it in a single statement outside the existing transaction.
By default, DROP COLUMN drops any indexes on the column being dropped, and any indexes that reference the column, including indexes with STORING clauses that reference the column.
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 you want to drop. |
name |
The name of the column you want to drop. When a column with a CHECK constraint is dropped, the CHECK constraint is also dropped. |
CASCADE |
Drop the column even if objects (such as views) depend on it; drop the dependent objects, as well. CASCADE will drop a column with a foreign key constraint if it is the only column in the reference.CASCADE does not list the objects it drops, so should be used cautiously.CASCADE is not required to drop an indexed column, or a column that is referenced by an index. By default, DROP COLUMN drops any indexes on the column being dropped, and any indexes that reference the column, including partial indexes with predicates that reference the column and indexes with STORING clauses that reference the column. |
RESTRICT |
(Default) Do not drop the column if any objects (such as views) depend on it. |
Viewing schema changes
This schema change statement is registered as a job. You can view long-running jobs with SHOW JOBS.
Known limitations
- CockroachDB prevents a column from being dropped if it is referenced by a partial index predicate. To drop such a column, the partial indexes need to be dropped first using
DROP INDEX. See tracking issue.
Examples
Setup
The following examples use MovR, a fictional vehicle-sharing application, to demonstrate CockroachDB SQL statements. For more information about the MovR example application and dataset, see MovR: A Global Vehicle-sharing App.
To follow along, run cockroach demo to start a temporary, in-memory cluster with the movr dataset preloaded:
$ cockroach demo
Drop a column
If you no longer want a column in a table, you can drop it.
> SHOW COLUMNS FROM users;
column_name | data_type | is_nullable | column_default | generation_expression | indices | is_hidden
--------------+-----------+-------------+----------------+-----------------------+-----------+------------
id | UUID | false | NULL | | {primary} | false
city | VARCHAR | false | NULL | | {primary} | false
name | VARCHAR | true | NULL | | {primary} | false
address | VARCHAR | true | NULL | | {primary} | false
credit_card | VARCHAR | true | NULL | | {primary} | false
(5 rows)
If there is data in the table, the sql_safe_updates session variable must be set to false.
> ALTER TABLE users DROP COLUMN credit_card;
ERROR: rejected (sql_safe_updates = true): ALTER TABLE DROP COLUMN will remove all data in that column
SQLSTATE: 01000
> SET sql_safe_updates = false;
> ALTER TABLE users DROP COLUMN credit_card;
> SHOW COLUMNS FROM users;
column_name | data_type | is_nullable | column_default | generation_expression | indices | is_hidden
--------------+-----------+-------------+----------------+-----------------------+-----------+------------
id | UUID | false | NULL | | {primary} | false
city | VARCHAR | false | NULL | | {primary} | false
name | VARCHAR | true | NULL | | {primary} | false
address | VARCHAR | true | NULL | | {primary} | false
(4 rows)
Prevent dropping columns with dependent objects (RESTRICT)
If the column has dependent objects, such as views, CockroachDB will not drop the column by default. However, if you want to be sure of the behavior you can include the RESTRICT clause.
> CREATE VIEW expensive_rides AS SELECT id, city FROM rides WHERE revenue > 90;
> ALTER TABLE rides DROP COLUMN revenue RESTRICT;
ERROR: cannot drop column "revenue" because view "expensive_rides" depends on it
SQLSTATE: 2BP01
HINT: you can drop expensive_rides instead.
Drop a column and its dependent objects (CASCADE)
If you want to drop the column and all of its dependent options, include the CASCADE clause.
CASCADE does not list objects it drops, so should be used cautiously.
> SHOW CREATE expensive_rides;
table_name | create_statement
------------------+-------------------------------------------------------------------------------------------------------------
expensive_rides | CREATE VIEW public.expensive_rides (id, city) AS SELECT id, city FROM movr.public.rides WHERE revenue > 90
(1 row)
> ALTER TABLE rides DROP COLUMN revenue CASCADE;
> SHOW CREATE expensive_rides;
ERROR: relation "expensive_rides" does not exist
SQLSTATE: 42P01
Drop an indexed column
DROP COLUMN drops a column and any indexes on the column being dropped.
> CREATE INDEX start_end_idx ON rides(start_time, end_time);
> WITH x AS (SHOW INDEXES FROM rides) SELECT * FROM x WHERE index_name='start_end_idx';
table_name | index_name | non_unique | seq_in_index | column_name | direction | storing | implicit
-------------+---------------+------------+--------------+-------------+-----------+---------+-----------
rides | start_end_idx | true | 1 | start_time | ASC | false | false
rides | start_end_idx | true | 2 | end_time | ASC | false | false
rides | start_end_idx | true | 3 | city | ASC | false | true
rides | start_end_idx | true | 4 | id | ASC | false | true
(4 rows)
> ALTER TABLE rides DROP COLUMN start_time;
NOTICE: the data for dropped indexes is reclaimed asynchronously
HINT: The reclamation delay can be customized in the zone configuration for the table.
ALTER TABLE
> WITH x AS (SHOW INDEXES FROM rides) SELECT * FROM x WHERE index_name='start_end_idx';
table_name | index_name | non_unique | seq_in_index | column_name | direction | storing | implicit
-------------+------------+------------+--------------+-------------+-----------+---------+-----------
(0 rows)
See also
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