DWH : Index on Partition Key s

Would there be any reason to create a single column index on the PARTITION_KEY column (which is a VARCHAR2 representation of a date --- I know, I know, bear with me !) ?
So, we have 24 PARTITION_KEY values for the 24 months that we are holding data.
Why have a single-column index on PARTITION_KEY ?

(no this isn't a trick question : I am really confused by this design that I see having been implemented).

Hemant K Chitale



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