Column Manipulations

A set of queries that allow changing the table structure.

Syntax:

ALTER [TEMPORARY] 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, TTL, and column settings.
• MODIFY COLUMN REMOVE — Removes one of the column properties.
• MODIFY COLUMN MODIFY SETTING - Changes column settings.
• MODIFY COLUMN RESET SETTING - Reset column settings.
• 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.

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 'This column shows the browser used for accessing the site.'

MODIFY COLUMN ​

MODIFY COLUMN [IF EXISTS] name [type] [default_expr] [codec] [TTL] [settings] [AFTER name_after | FIRST]
ALTER COLUMN [IF EXISTS] name TYPE [type] [default_expr] [codec] [TTL] [settings] [AFTER name_after | FIRST]

This query changes the name column properties:

• Type

• Default expression

• Compression Codec

• TTL

• Column-level Settings

For examples of columns compression CODECS modifying, see Column Compression Codecs .

For examples of columns TTL modifying, see Column TTL .

For examples of column-level settings modifying, see Column-level Settings .

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 , SETTINGS .

Syntax:

ALTER TABLE table_name MODIFY COLUMN column_name REMOVE property;

Example

Remove TTL:

ALTER TABLE table_with_ttl MODIFY COLUMN column_ttl REMOVE TTL;

See Also

• REMOVE TTL .

MODIFY COLUMN MODIFY SETTING ​

Modify a column setting.

Syntax:

ALTER TABLE table_name MODIFY COLUMN column_name MODIFY SETTING name=value,...;

Example

Modify column's max_compress_block_size to 1MB :

ALTER TABLE table_name MODIFY COLUMN column_name MODIFY SETTING max_compress_block_size = 1048576;

MODIFY COLUMN RESET SETTING ​

Reset a column setting, also removes the setting declaration in the column expression of the table's CREATE query.

Syntax:

ALTER TABLE table_name MODIFY COLUMN column_name RESET SETTING name,...;

Example

Reset column setting max_compress_block_size to it's default value:

ALTER TABLE table_name MODIFY COLUMN column_name RESET SETTING max_compress_block_size;

MATERIALIZE COLUMN ​

Materializes a column with a DEFAULT or MATERIALIZED value expression.