thevar1able/ClickHouse
1
0
Fork 0
mirror of https://github.com/ClickHouse/ClickHouse.git synced 2024-11-21 15:12:02 +00:00
Code Issues Packages Projects Releases Wiki Activity

Update count.md

Browse Source
This commit is contained in:
Anton Popov 2021-06-30 14:25:14 +03:00 committed by GitHub
parent ca57d62f70
commit d9fcd53f0c
No known key found for this signature in database
GPG Key ID: 4AEE18F83AFDEB23
1 changed files with 1 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
docs/en/sql-reference/aggregate-functions/reference/count.md
View File

@ -31,7 +31,7 @@ ClickHouse supports the `COUNT(DISTINCT ...)` syntax. The behavior of this const
The `SELECT count() FROM table` query is not optimized, because the number of entries in the table is not stored separately. It chooses a small column from the table and counts the number of values in it.
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 table data. The query `SELECT count(n) FROM table` transforms to `SELECT sum(NOT n.null) FROM table`.
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`.
**Examples**