Update Mutations
Update mutations refers to ALTER queries that manipulate table data through updates. Most notably they are queries like ALTER TABLE UPDATE, etc. Performing such queries will produce new mutated versions of the data parts. This means that such statements would trigger a rewrite of whole data parts for all data that was inserted before the mutation, translating to a large amount of write requests.
For updates, you can avoid these large amounts of write requests by using specialised table engines like ReplacingMergeTree or CollapsingMergeTree instead of the default MergeTree table engine.
ALTER TABLE ... UPDATE Statements
ALTER TABLE [db.]table [ON CLUSTER cluster] UPDATE column1 = expr1 [, ...] [IN PARTITION partition_id] WHERE filter_expr
Manipulates data matching the specified filtering expression. Implemented as a mutation.
The ALTER TABLE prefix makes this syntax different from most other systems supporting SQL. It is intended to signify that unlike similar queries in OLTP databases this is a heavy operation not designed for frequent use.
The filter_expr must be of type UInt8. This query updates values of specified columns to the values of corresponding expressions in rows for which the filter_expr takes a non-zero value. Values are cast to the column type using the CAST operator. Updating columns that are used in the calculation of the primary or the partition key is not supported.
One query can contain several commands separated by commas.
The synchronicity of the query processing is defined by the mutations_sync setting. By default, it is asynchronous.
See also