ClickHouse/docs/en/sql-reference/statements/alter/sample-by.md

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41	SAMPLE BY

Manipulating Sampling-Key Expressions

Syntax:

ALTER TABLE [db].name [ON CLUSTER cluster] MODIFY SAMPLE BY new_expression

The command changes the sampling key of the table to new_expression (an expression or a tuple of expressions).

The command is lightweight in the sense that it only changes metadata. The primary key must contain the new sample key.

!!! note "Note" It only works for tables in the MergeTree family (including replicated tables).