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Updating and Deleting ClickHouse Data

Although ClickHouse is geared toward high volume analytic workloads, it is possible in some situations to modify or delete existing data. These operations are labeled "mutations" and are executed using the ALTER TABLE command. You can also DELETE a row using the lightweight delete capability of ClickHouse.

tip

If you need to perform frequent updates, consider using deduplication in ClickHouse, which allows you to update and/or delete rows without generating a mutation event.

Updating Data​

Use the ALTER TABLE...UPDATE command to update rows in a table:

ALTER TABLE [<database>.]<table> UPDATE <column> = <expression> WHERE <filter_expr>

<expression> is the new value for the column where the <filter_expr> is satisfied. The <expression> must be the same datatype as the column or be convertable to the same datatype using the CAST operator. The <filter_expr> should return a UInt8 (zero or non-zero) value for each row of the data. Multiple UPDATE <column> statements can be combined in a single ALTER TABLE command separated by commas.

Examples:

  1. A mutation like this allows updating replacing visitor_ids with new ones using a dictionary lookup:

    ALTER TABLE website.clicks
    UPDATE visitor_id = getDict('visitors', 'new_visitor_id', visitor_id)
    WHERE visit_date < '2022-01-01'

  2. Modifying multiple values in one command can be more efficient than multiple commands:

    ALTER TABLE website.clicks
    UPDATE url = substring(url, position(url, '://') + 3), visitor_id = new_visit_id
    WHERE visit_date < '2022-01-01'

  3. Mutations can be exectuted ON CLUSTER for sharded tables:

    ALTER TABLE clicks ON CLUSTER main_cluster
    UPDATE click_count = click_count / 2
    WHERE visitor_id ILIKE '%robot%'
note

It is not possible to update columns that are part of the primary or sorting key.

Deleting Data​

Use the ALTER TABLE command to delete rows:

ALTER TABLE [<database>.]<table> DELETE WHERE <filter_expr>

The <filter_expr> should return a UInt8 value for each row of data.

Examples

  1. Delete any records where a column is in an array of values:

    ALTER TABLE website.clicks DELETE WHERE visitor_id in (253, 1002, 4277)

  2. What does this query alter?

    ALTER TABLE clicks ON CLUSTER main_cluster DELETE WHERE visit_date < '2022-01-02 15:00:00' AND page_id = '573'
note

To delete all of the data in a table, it is more efficient to use the command TRUNCATE TABLE [<database].]<table> command. This command can also be executed ON CLUSTER.

View the DELETE statement docs page for more details.

Lightweight Deletes​

Another option for deleting rows it to use the DELETE FROM command, which is referred to as a lightweight delete. The deleted rows are marked as deleted immediately and will be automatically filtered out of all subsequent queries, so you do not have to wait for a merging of parts or use the FINAL keyword. Cleanup of data happens asynchronously in the background.

DELETE FROM [db.]table [ON CLUSTER cluster] [WHERE expr]

For example, the following query deletes all rows from the hits table where the Title column contains the text hello:

DELETE FROM hits WHERE Title LIKE '%hello%';

A few notes about lightweight deletes:

  • This feature is only available for the MergeTree table engine family.
  • Lightweight deletes are asynchronous by default. Set mutations_sync equal to 1 to wait for one replica to process the statement, and set mutations_sync to 2 to wait for all replicas.
  • This feature is experimental and requires you to set allow_experimental_lightweight_delete to true:
SET allow_experimental_lightweight_delete = true;