[PATCH] distinct aggregates within a window function WIP

Hi. I read the thread.

Probably this fiddle will be helpful for testing:

https://dbfiddle.uk/?rdbms=postgres_12&fiddle=abe845142a5099d921d3729043fb8491

I recently encountered a problem:
Why Window-specific functions do not allow DISTINCT to be used within the function argument list?

sum( DISTINCT order_cost ) OVER ( PARTITION BY invoice_id ORDER BY invoice_id, group_id RANGE unbound preceeding and unbound following )

behavior is quite deterministic:

ORDER BY will create peers in partition
DISTINCT will get only one peer

I resolve my problem via two subqueries, but it seems this logic may
be applied to window functions (did not check this for other functions thought)

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Best regards,
Eugen Konkov