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rfraposa 2022-09-09 13:59:21 -06:00
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5
docs/en/sql-reference/statements/alter/delete.md
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@ -12,12 +12,9 @@ ALTER TABLE [db.]table [ON CLUSTER cluster] DELETE WHERE filter_expr
Deletes data matching the specified filtering expression. Implemented as a [mutation](../../../sql-reference/statements/alter/index.md#mutations).
:::info
The `ALTER TABLE` command is considered a heavyweight operation that requires the underlying data to be merged before it is deleted. For MergeTree tables, consider using the [`DELETE FROM` query](../delete.md), which performs a lightweight delete and can be considerably faster.
:::
:::note
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 `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. `ALTER TABLE` is considered a heavyweight operation that requires the underlying data to be merged before it is deleted. For MergeTree tables, consider using the [`DELETE FROM` query](../delete.md), which performs a lightweight delete and can be considerably faster.
:::
The `filter_expr` must be of type `UInt8`. The query deletes rows in the table for which this expression takes a non-zero value.

5
docs/en/sql-reference/statements/delete.md
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@ -18,6 +18,7 @@ For example, the following query deletes all rows from the `hits` table where th
DELETE FROM hits WHERE Title LIKE '%hello%';
```
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.
:::note
This feature is experimental and requires you to set `allow_experimental_lightweight_delete` to true:
@ -30,7 +31,7 @@ SET allow_experimental_lightweight_delete = true;
An [alternative way to delete rows](./alter/delete.md) in ClickHouse is `ALTER TABLE ... DELETE`, which might be more efficient if you do bulk deletes only occasionally and don't need the operation to be applied instantly. In most use cases the new lightweight `DELETE FROM` behavior will be considerably faster.
:::info
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.
:::warn
Even though deletes are becoming more lightweight in ClickHouse, they should still not be used as aggressively as on OLTP system. Ligthweight deletes are currently efficient for wide parts, but for compact parts they can be a heavyweight operation, and it may be better to use `ALTER TABLE` for some scenarios.
:::