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slug | sidebar_position | sidebar_label | title |
---|---|---|---|
/en/sql-reference/statements/alter/column | 37 | COLUMN | Column Manipulations |
A set of queries that allow changing the table structure.
Syntax:
ALTER TABLE [db].name [ON CLUSTER cluster] ADD|DROP|RENAME|CLEAR|COMMENT|{MODIFY|ALTER}|MATERIALIZE COLUMN ...
In the query, specify a list of one or more comma-separated actions. Each action is an operation on a column.
The following actions are supported:
- ADD COLUMN — Adds a new column to the table.
- DROP COLUMN — Deletes the column.
- RENAME COLUMN — Renames an existing column.
- CLEAR COLUMN — Resets column values.
- COMMENT COLUMN — Adds a text comment to the column.
- MODIFY COLUMN — Changes column’s type, default expression and TTL.
- MODIFY COLUMN REMOVE — Removes one of the column properties.
- MATERIALIZE COLUMN — Materializes the column in the parts where the column is missing.
These actions are described in detail below.
ADD COLUMN
ADD COLUMN [IF NOT EXISTS] name [type] [default_expr] [codec] [AFTER name_after | FIRST]
Adds a new column to the table with the specified name
, type
, codec
and default_expr
(see the section Default expressions).
If the IF NOT EXISTS
clause is included, the query won’t return an error if the column already exists. If you specify AFTER name_after
(the name of another column), the column is added after the specified one in the list of table columns. If you want to add a column to the beginning of the table use the FIRST
clause. Otherwise, the column is added to the end of the table. For a chain of actions, name_after
can be the name of a column that is added in one of the previous actions.
Adding a column just changes the table structure, without performing any actions with data. The data does not appear on the disk after ALTER
. If the data is missing for a column when reading from the table, it is filled in with default values (by performing the default expression if there is one, or using zeros or empty strings). The column appears on the disk after merging data parts (see MergeTree).
This approach allows us to complete the ALTER
query instantly, without increasing the volume of old data.
Example:
ALTER TABLE alter_test ADD COLUMN Added1 UInt32 FIRST;
ALTER TABLE alter_test ADD COLUMN Added2 UInt32 AFTER NestedColumn;
ALTER TABLE alter_test ADD COLUMN Added3 UInt32 AFTER ToDrop;
DESC alter_test FORMAT TSV;
Added1 UInt32
CounterID UInt32
StartDate Date
UserID UInt32
VisitID UInt32
NestedColumn.A Array(UInt8)
NestedColumn.S Array(String)
Added2 UInt32
ToDrop UInt32
Added3 UInt32
DROP COLUMN
DROP COLUMN [IF EXISTS] name
Deletes the column with the name name
. If the IF EXISTS
clause is specified, the query won’t return an error if the column does not exist.
Deletes data from the file system. Since this deletes entire files, the query is completed almost instantly.
:::tip
You can’t delete a column if it is referenced by materialized view. Otherwise, it returns an error.
:::
Example:
ALTER TABLE visits DROP COLUMN browser
RENAME COLUMN
RENAME COLUMN [IF EXISTS] name to new_name
Renames the column name
to new_name
. If the IF EXISTS
clause is specified, the query won’t return an error if the column does not exist. Since renaming does not involve the underlying data, the query is completed almost instantly.
NOTE: Columns specified in the key expression of the table (either with ORDER BY
or PRIMARY KEY
) cannot be renamed. Trying to change these columns will produce SQL Error [524]
.
Example:
ALTER TABLE visits RENAME COLUMN webBrowser TO browser
CLEAR COLUMN
CLEAR COLUMN [IF EXISTS] name IN PARTITION partition_name
Resets all data in a column for a specified partition. Read more about setting the partition name in the section How to set the partition expression.
If the IF EXISTS
clause is specified, the query won’t return an error if the column does not exist.
Example:
ALTER TABLE visits CLEAR COLUMN browser IN PARTITION tuple()
COMMENT COLUMN
COMMENT COLUMN [IF EXISTS] name 'Text comment'
Adds a comment to the column. If the IF EXISTS
clause is specified, the query won’t return an error if the column does not exist.
Each column can have one comment. If a comment already exists for the column, a new comment overwrites the previous comment.
Comments are stored in the comment_expression
column returned by the DESCRIBE TABLE query.
Example:
ALTER TABLE visits COMMENT COLUMN browser 'The table shows the browser used for accessing the site.'
MODIFY COLUMN
MODIFY COLUMN [IF EXISTS] name [type] [default_expr] [codec] [TTL] [AFTER name_after | FIRST]
ALTER COLUMN [IF EXISTS] name TYPE [type] [default_expr] [codec] [TTL] [AFTER name_after | FIRST]
This query changes the name
column properties:
-
Type
-
Default expression
-
Compression Codec
-
TTL
For examples of columns compression CODECS modifying, see Column Compression Codecs.
For examples of columns TTL modifying, see Column TTL.
If the IF EXISTS
clause is specified, the query won’t return an error if the column does not exist.
When changing the type, values are converted as if the toType functions were applied to them. If only the default expression is changed, the query does not do anything complex, and is completed almost instantly.
Example:
ALTER TABLE visits MODIFY COLUMN browser Array(String)
Changing the column type is the only complex action – it changes the contents of files with data. For large tables, this may take a long time.
The query also can change the order of the columns using FIRST | AFTER
clause, see ADD COLUMN description, but column type is mandatory in this case.
Example:
CREATE TABLE users (
c1 Int16,
c2 String
) ENGINE = MergeTree
ORDER BY c1;
DESCRIBE users;
┌─name─┬─type───┬
│ c1 │ Int16 │
│ c2 │ String │
└──────┴────────┴
ALTER TABLE users MODIFY COLUMN c2 String FIRST;
DESCRIBE users;
┌─name─┬─type───┬
│ c2 │ String │
│ c1 │ Int16 │
└──────┴────────┴
ALTER TABLE users ALTER COLUMN c2 TYPE String AFTER c1;
DESCRIBE users;
┌─name─┬─type───┬
│ c1 │ Int16 │
│ c2 │ String │
└──────┴────────┴
The ALTER
query is atomic. For MergeTree tables it is also lock-free.
The ALTER
query for changing columns is replicated. The instructions are saved in ZooKeeper, then each replica applies them. All ALTER
queries are run in the same order. The query waits for the appropriate actions to be completed on the other replicas. However, a query to change columns in a replicated table can be interrupted, and all actions will be performed asynchronously.
MODIFY COLUMN REMOVE
Removes one of the column properties: DEFAULT
, ALIAS
, MATERIALIZED
, CODEC
, COMMENT
, TTL
.
Syntax:
ALTER TABLE table_name MODIFY column_name REMOVE property;
Example
Remove TTL:
ALTER TABLE table_with_ttl MODIFY COLUMN column_ttl REMOVE TTL;
See Also
MATERIALIZE COLUMN
Materializes or updates a column with an expression for a default value (DEFAULT
or MATERIALIZED
).
It is used if it is necessary to add or update a column with a complicated expression, because evaluating such an expression directly on SELECT
executing turns out to be expensive.
Syntax:
ALTER TABLE [db.]table [ON CLUSTER cluster] MATERIALIZE COLUMN col [IN PARTITION partition | IN PARTITION ID 'partition_id'];
- If you specify a PARTITION, a column will be materialized with only the specified partition.
Example
DROP TABLE IF EXISTS tmp;
SET mutations_sync = 2;
CREATE TABLE tmp (x Int64) ENGINE = MergeTree() ORDER BY tuple() PARTITION BY tuple();
INSERT INTO tmp SELECT * FROM system.numbers LIMIT 5;
ALTER TABLE tmp ADD COLUMN s String MATERIALIZED toString(x);
ALTER TABLE tmp MATERIALIZE COLUMN s;
SELECT groupArray(x), groupArray(s) FROM (select x,s from tmp order by x);
┌─groupArray(x)─┬─groupArray(s)─────────┐
│ [0,1,2,3,4] │ ['0','1','2','3','4'] │
└───────────────┴───────────────────────┘
ALTER TABLE tmp MODIFY COLUMN s String MATERIALIZED toString(round(100/x));
INSERT INTO tmp SELECT * FROM system.numbers LIMIT 5,5;
SELECT groupArray(x), groupArray(s) FROM tmp;
┌─groupArray(x)─────────┬─groupArray(s)──────────────────────────────────┐
│ [0,1,2,3,4,5,6,7,8,9] │ ['0','1','2','3','4','20','17','14','12','11'] │
└───────────────────────┴────────────────────────────────────────────────┘
ALTER TABLE tmp MATERIALIZE COLUMN s;
SELECT groupArray(x), groupArray(s) FROM tmp;
┌─groupArray(x)─────────┬─groupArray(s)─────────────────────────────────────────┐
│ [0,1,2,3,4,5,6,7,8,9] │ ['inf','100','50','33','25','20','17','14','12','11'] │
└───────────────────────┴───────────────────────────────────────────────────────┘
See Also
Limitations
The ALTER
query lets you create and delete separate elements (columns) in nested data structures, but not whole nested data structures. To add a nested data structure, you can add columns with a name like name.nested_name
and the type Array(T)
. A nested data structure is equivalent to multiple array columns with a name that has the same prefix before the dot.
There is no support for deleting columns in the primary key or the sampling key (columns that are used in the ENGINE
expression). Changing the type for columns that are included in the primary key is only possible if this change does not cause the data to be modified (for example, you are allowed to add values to an Enum or to change a type from DateTime
to UInt32
).
If the ALTER
query is not sufficient to make the table changes you need, you can create a new table, copy the data to it using the INSERT SELECT query, then switch the tables using the RENAME query and delete the old table. You can use the clickhouse-copier as an alternative to the INSERT SELECT
query.
The ALTER
query blocks all reads and writes for the table. In other words, if a long SELECT
is running at the time of the ALTER
query, the ALTER
query will wait for it to complete. At the same time, all new queries to the same table will wait while this ALTER
is running.
For tables that do not store data themselves (such as Merge and Distributed), ALTER
just changes the table structure, and does not change the structure of subordinate tables. For example, when running ALTER for a Distributed
table, you will also need to run ALTER
for the tables on all remote servers.