add documentation for getSubcolumn and getTypeSerializationStreams

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

@@ -4055,3 +4055,94 @@ Result:
 │                          32 │
 └─────────────────────────────┘
 ```
+
+## getSubcolumn
+
+Takes a table expression or identifier and constant string with the name of the sub-column, and returns the requested sub-column extracted from the expression.
+
+**Syntax**
+
+```sql
+getSubcolumn(col_name, subcol_name)
+```
+
+**Arguments**
+
+- `col_name` — Table expression or identifier. [Expression](../syntax.md/#expressions), [Identifier](../syntax.md/#identifiers).
+- `subcol_name` — The name of the sub-column. [String](../data-types/string.md).
+
+**Returned value**
+
+- Returns the extracted sub-colum.
+
+**Example**
+
+Query:
+
+```sql
+CREATE TABLE t_arr (arr Array(Tuple(subcolumn1 UInt32, subcolumn2 String))) ENGINE = MergeTree ORDER BY tuple();
+INSERT INTO t_arr VALUES ([(1, 'Hello'), (2, 'World')]), ([(3, 'This'), (4, 'is'), (5, 'subcolumn')]);
+SELECT getSubcolumn(arr, 'subcolumn1'), getSubcolumn(arr, 'subcolumn2') FROM t_arr;
+```
+
+Result:
+
+```response
+   ┌─getSubcolumn(arr, 'subcolumn1')─┬─getSubcolumn(arr, 'subcolumn2')─┐
+1. │ [1,2]                           │ ['Hello','World']               │
+2. │ [3,4,5]                         │ ['This','is','subcolumn']       │
+   └─────────────────────────────────┴─────────────────────────────────┘
+```
+
+## getTypeSerializationStreams
+
+Enumerates stream paths of a data type.
+
+:::note
+This function is intended for use by developers.
+:::
+
+**Syntax**
+
+```sql
+getTypeSerializationStreams(col)
+```
+
+**Arguments**
+
+- `col` — Column or string representation of a data-type from which the data type will be detected.
+
+**Returned value**
+
+- Returns an array with all the serialization sub-stream paths.[Array](../data-types/array.md)([String](../data-types/string.md)).
+
+**Examples**
+
+Query:
+
+```sql
+SELECT getTypeSerializationStreams(tuple('a', 1, 'b', 2));
+```
+
+Result:
+
+```response
+   ┌─getTypeSerializationStreams(('a', 1, 'b', 2))─────────────────────────────────────────────────────────────────────────┐
+1. │ ['{TupleElement(1), Regular}','{TupleElement(2), Regular}','{TupleElement(3), Regular}','{TupleElement(4), Regular}'] │
+   └───────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
+```
+
+Query:
+
+```sql
+SELECT getTypeSerializationStreams('Map(String, Int64)');
+```
+
+Result:
+
+```response
+   ┌─getTypeSerializationStreams('Map(String, Int64)')────────────────────────────────────────────────────────────────┐
+1. │ ['{ArraySizes}','{ArrayElements, TupleElement(keys), Regular}','{ArrayElements, TupleElement(values), Regular}'] │
+   └──────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
+```
+