Window function with valued based range frames?

ex. avg(value) over (order by date range interval '6' day preceding)

Source:
http://wwwlgis.informatik.uni-kl.de/cms/fileadmin/courses/SS2008/NEDM/RDDM.Chapter.06.Windows_and_Query_Functions_in_SQL.pdf

Page 9, slide 17

The SELECT SQL command page mentions standard aspects of window frame
clauses not being supported but is not specific (supposedly in order to
having people ask for these things).

Just looking for recollection regarding why these were omitted initially and
if anyone has concerned adding them in follow-up. With the recent
hypothetical work being done maybe these can be re-evaluated in a fresh
light? They (whatever they is) are standard and do seem generally useful.

I don't personally have an immediate need but have been pondering moving
average related window queries and how performant they are in PostgreSQL
version possible alternative calculation means and came across this
presentation.

David J.

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