thevar1able/ClickHouse
1
0
Fork 0
mirror of https://github.com/ClickHouse/ClickHouse.git synced 2024-11-22 07:31:57 +00:00
Code Issues Packages Projects Releases Wiki Activity

[Docs] Add perf tip for COUNT(DISTINCT expr)

Browse Source
This commit is contained in:
Justin de Guzman 2023-11-18 17:53:32 -08:00 committed by GitHub
parent c465933b1d
commit a894671e8a
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 4 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

4
docs/en/sql-reference/aggregate-functions/reference/count.md
View File

@ -34,6 +34,10 @@ The `SELECT count() FROM table` query is optimized by default using metadata fro
However `SELECT count(nullable_column) FROM table` query can be optimized by enabling the [optimize_functions_to_subcolumns](../../../operations/settings/settings.md#optimize-functions-to-subcolumns) setting. With `optimize_functions_to_subcolumns = 1` the function reads only [null](../../../sql-reference/data-types/nullable.md#finding-null) subcolumn instead of reading and processing the whole column data. The query `SELECT count(n) FROM table` transforms to `SELECT sum(NOT n.null) FROM table`.
**Improving COUNT(DISTINCT expr) performance**
If your `COUNT(DISTINCT expr)` query is slow, consider adding a [`GROUP BY`](../../../sql-reference/statements/select/group-by.md) clause as this improves parallelization. You can also use a [projection](../../../sql-reference/statements/alter/projection.md) to create an index on the target column used with `COUNT(DISTINCT target_col)`.
**Examples**
Example 1: