Add _partition_id Virtual Column and partitionID() Function for MergeTree Engines

Why it matters

This feature allows users to efficiently filter and prune data partitions by using the _partition_id virtual column in queries. It improves query performance when working with partitioned tables by enabling direct access to partition identifiers. The partitionID() function provides a way to programmatically obtain the partition ID string for specific rows or data.

How to use it

Users can use the _partition_id virtual column directly in the WHERE clause to filter partitions in SELECT, ALTER, and other queries on MergeTree* tables. The partitionID() function can be called in queries to get the partition ID string for a row. Example usage:

SELECT * FROM table WHERE _partition_id = '2023-06-01';

SELECT partitionID() FROM table LIMIT 1;