Support Subcolumns in MergeTree Sorting Key and Skip Indexes

Why it matters

This feature enables users to define sorting keys and skip indexes on subcolumns within nested data structures in MergeTree tables. It improves query efficiency and storage optimization by allowing more granular control over complex data types.

How to use it

Users can specify subcolumns in the sorting key and skip indexes definitions when creating or altering MergeTree tables using standard DDL syntax. For example, you can reference nested fields in the ORDER BY clause or skip index definitions.