En docs

docs/en/operations/settings/settings.md

@@ -1729,16 +1729,25 @@ Default value: 0.
 
 ## optimize_functions_to_subcolumns {#optimize-functions-to-subcolumns}
 
-Optimizes functions (if possible) to subcolumns to reduce amount of read data. 
+Optimizes functions (if possible) by transforming them to read the subcolumns. This reduces the amount of read data. 
 
--   
+These function can be tranformed:
+
+-   [length](../../sql-reference/functions/array-functions.md#array_functions-length) to read subcolumn [size0](../../sql-reference/data-types/array.md#array-size).
+-   [empty](../../sql-reference/functions/array-functions.md#function-empty) to read subcolumn [size0](../../sql-reference/data-types/array.md#array-size).
+-   [notEmpty](../../sql-reference/functions/array-functions.md#function-notempty) to read subcolumn [size0](../../sql-reference/data-types/array.md#array-size).
+-   [isNull](../../sql-reference/operators/index.md#operator-is-null) to read subcolumn [null](../../sql-reference/data-types/nullable.md#finding-null).
+-   [isNotNull](../../sql-reference/operators.md#is-not-null#is-not-null) to read subcolumn [null](../../sql-reference/data-types/nullable.md#finding-null).
+-   [count](../../sql-reference/aggregate-functions/reference/count.md) to read subcolumn [null](../../sql-reference/data-types/nullable.md#finding-null).
+-   [mapKeys](../../sql-reference/functions/tuple-map-functions.md#mapkeys) to read subcolumn [keys](../../sql-reference/data-types/map.md#subcolumn-keys).
+-   [mapValues](../../sql-reference/functions/tuple-map-functions.md#mapvalues) to read subcolumn [values](../../sql-reference/data-types/map.md#subcolumn-values).
 
 Possible values:
 
 -   0 — Disabled.
 -   1 — Enabled.
 
-Default value: `value`.
+Default value: `0`.
 
 ## distributed_replica_error_half_life {#settings-distributed_replica_error_half_life}
 

docs/en/sql-reference/aggregate-functions/reference/count.md

@@ -31,6 +31,8 @@ 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.
 
+Can be optimized by setting [optimize_functions_to_subcolumns](../../operations/settings/settings.md#optimize-functions-to-subcolumns). With `optimize_functions_to_subcolumns = 1` the function reads subcolumn [null](../../sql-reference/data-types/nullable.md#finding-null).
+
 **Examples**
 
 Example 1:

docs/en/sql-reference/data-types/map.md

@@ -75,6 +75,54 @@ SELECT CAST(([1, 2, 3], ['Ready', 'Steady', 'Go']), 'Map(UInt8, String)') AS map
 └───────────────────────────────┘
 ```
 
+## Subcolumn Map.keys {#subcolumn-keys}
+
+To read all keys of a `Map` you can use the subcolumn `keys`, which doesn't read the whole column.
+
+**Example**
+
+Query:
+
+``` sql
+CREATE TABLE t_map (`a` Map(String, UInt64)) ENGINE = Memory;
+
+INSERT INTO t_map VALUES (map('key1', 1, 'key2', 2, 'key3', 3));
+
+SELECT a.keys FROM t_map;
+```
+
+Result:
+
+``` text
+┌─a.keys─────────────────┐
+│ ['key1','key2','key3'] │
+└────────────────────────┘
+```
+
+## Subcolumn Map.values {#subcolumn-keys}
+
+To read all values of a `Map` you can use the subcolumn `values`, which doesn't read the whole column.
+
+**Example**
+
+Query:
+
+``` sql
+CREATE TABLE t_map (`a` Map(String, UInt64)) ENGINE = Memory;
+
+INSERT INTO t_map VALUES (map('key1', 1, 'key2', 2, 'key3', 3))
+
+SELECT a.values FROM t_map;
+```
+
+Result:
+
+``` text
+┌─a.values─┐
+│ [1,2,3]  │
+└──────────┘
+```
+
 **See Also**
 
 -   [map()](../../sql-reference/functions/tuple-map-functions.md#function-map) function

docs/en/sql-reference/functions/array-functions.md

@@ -11,18 +11,24 @@ Returns 1 for an empty array, or 0 for a non-empty array.
 The result type is UInt8.
 The function also works for strings.
 
+Can be optimized by setting [optimize_functions_to_subcolumns](../../operations/settings/settings.md#optimize-functions-to-subcolumns). With `optimize_functions_to_subcolumns = 1` the function reads subcolumn [size0](../../sql-reference/data-types/array.md#array-size).
+
 ## notEmpty {#function-notempty}
 
 Returns 0 for an empty array, or 1 for a non-empty array.
 The result type is UInt8.
 The function also works for strings.
 
+Can be optimized by setting [optimize_functions_to_subcolumns](../../operations/settings/settings.md#optimize-functions-to-subcolumns). With `optimize_functions_to_subcolumns = 1` the function reads subcolumn [size0](../../sql-reference/data-types/array.md#array-size).
+
 ## length {#array_functions-length}
 
 Returns the number of items in the array.
 The result type is UInt64.
 The function also works for strings.
 
+Can be optimized by setting [optimize_functions_to_subcolumns](../../operations/settings/settings.md#optimize-functions-to-subcolumns). With `optimize_functions_to_subcolumns = 1` the function reads subcolumn [size0](../../sql-reference/data-types/array.md#array-size).
+
 ## emptyArrayUInt8, emptyArrayUInt16, emptyArrayUInt32, emptyArrayUInt64 {#emptyarrayuint8-emptyarrayuint16-emptyarrayuint32-emptyarrayuint64}
 
 ## emptyArrayInt8, emptyArrayInt16, emptyArrayInt32, emptyArrayInt64 {#emptyarrayint8-emptyarrayint16-emptyarrayint32-emptyarrayint64}

docs/en/sql-reference/functions/tuple-map-functions.md

@@ -220,6 +220,8 @@ Result:
 
 Returns all keys from the `map` parameter.
 
+Can be optimized by setting [optimize_functions_to_subcolumns](../../operations/settings/settings.md#optimize-functions-to-subcolumns). With `optimize_functions_to_subcolumns = 1` the function reads subcolumn [keys](../../sql-reference/data-types/map.md#subcolumn-keys).
+
 **Syntax**
 
 ```sql
@@ -261,6 +263,8 @@ Result:
 
 Returns all values from the `map` parameter.
 
+Can be optimized by setting [optimize_functions_to_subcolumns](../../operations/settings/settings.md#optimize-functions-to-subcolumns). With `optimize_functions_to_subcolumns = 1` the function reads subcolumn [values](../../sql-reference/data-types/map.md#subcolumn-values).
+
 **Syntax**
 
 ```sql

docs/en/sql-reference/operators/index.md

@@ -283,6 +283,8 @@ ClickHouse supports the `IS NULL` and `IS NOT NULL` operators.
     -   `0` otherwise.
 -   For other values, the `IS NULL` operator always returns `0`.
 
+Can be optimized by setting [optimize_functions_to_subcolumns](../../operations/settings/settings.md#optimize-functions-to-subcolumns). With `optimize_functions_to_subcolumns = 1` the function reads subcolumn [null](../../sql-reference/data-types/nullable.md#finding-null).
+
 <!-- -->
 
 ``` sql
@@ -313,3 +315,5 @@ SELECT * FROM t_null WHERE y IS NOT NULL
 │ 2 │ 3 │
 └───┴───┘
 ```
+
+Can be optimized by setting [optimize_functions_to_subcolumns](../../operations/settings/settings.md#optimize-functions-to-subcolumns). With `optimize_functions_to_subcolumns = 1` the function reads subcolumn [null](../../sql-reference/data-types/nullable.md#finding-null).