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Deletes data matching the specified filtering expression. Implemented as a mutation.
The ALTER TABLE prefix signals that this is a heavyweight operation, unlike similar queries in OLTP databases. ALTER TABLE ... DELETE rewrites every data part containing rows that match the filtering expression, generating substantial write I/O. Use it sparingly, as it can affect SELECT performance.For MergeTree tables, consider using the DELETE FROM query, which performs a lightweight delete and can be considerably faster. For workloads that require frequent deletions or corrections, consider using ReplacingMergeTree or CollapsingMergeTree.
The filter_expr must be of type UInt8. The query deletes rows in the table for which this expression takes a non-zero value. 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

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Last modified on July 27, 2026