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Document the optimize_trivial_count_query setting

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Tatiana Kirillova 2021-09-23 21:33:14 +03:00
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14
docs/en/operations/settings/settings.md
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@ -1933,6 +1933,20 @@ Possible values:
Default value: `0`.
## optimize_trivial_count_query (optimize-trivial-count-query)
Enables or disables optimization by metadata for trivial query `SELECT count() FROM table`. If row-level security is set, disable trivial count query optimization.
Possible values:
- 0 — Optimization disabled.
- 1 — Optimization enabled.
Default value: `1`.
**See also:**
- [optimize_functions_to_subcolumns](#optimize-trivial-count-query)
## distributed_replica_error_half_life {#settings-distributed_replica_error_half_life}
- Type: seconds

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docs/en/sql-reference/aggregate-functions/reference/count.md
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@ -29,7 +29,7 @@ In both cases the type of the returned value is [UInt64](../../../sql-reference/
ClickHouse supports the `COUNT(DISTINCT ...)` syntax. The behavior of this construction depends on the [count_distinct_implementation](../../../operations/settings/settings.md#settings-count_distinct_implementation) setting. It defines which of the [uniq\*](../../../sql-reference/aggregate-functions/reference/uniq.md#agg_function-uniq) functions is used to perform the operation. The default is the [uniqExact](../../../sql-reference/aggregate-functions/reference/uniqexact.md#agg_function-uniqexact) function.
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.
The `SELECT count() FROM table` query is optimized by default with metadata. If you need to use row-level security, disable [optimize_trivial_count_query](../../../operations/settings/settings.md#optimize_trivial_count_query) setting by set `optimize_trivial_count_query = 0`.
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`.