Which of the following statements should be used to drop a column from the table?

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The DROP COLUMN statement is part of ALTER TABLE and removes columns from a table.

Note:

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

Warning:

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.

Note:

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.

Synopsis

ALTERTABLEIFEXISTStable_nameDROP COLUMNIFEXISTSnameCASCADERESTRICT

Required privileges

The user must have the CREATE privilege on the table.

Parameters

Viewing schema changes

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

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:

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Drop a column

If you no longer want a column in a table, you can drop it.

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> 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.

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> 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

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> SET sql_safe_updates = false;

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> ALTER TABLE users DROP COLUMN credit_card;

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> 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.

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> CREATE VIEW expensive_rides AS SELECT id, city FROM rides WHERE revenue > 90;

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> 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.

Warning:

CASCADE does not list objects it drops, so should be used cautiously.

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> 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)

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> ALTER TABLE rides DROP COLUMN revenue CASCADE;

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> 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.

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> CREATE INDEX start_end_idx ON rides(start_time, end_time);

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> 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)

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> 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

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> 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

• DROP CONSTRAINT
• DROP INDEX
• ALTER TABLE
• SHOW JOBS
• Online Schema Changes

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