How do window functions and the group by clause interact?

Question

I do understand window functions and group by separately.

But what happens when you use both a window function and a group by clause in the same query ?

• Are the selected rows grouped first, then considered by the window function ?
• Or does the window function executes first, then the resulting values are grouped by group by ?
• Something else ?

Answer 1

Quote from the manual:

If the query contains any window functions, these functions are evaluated after any grouping, aggregation, and HAVING filtering is performed

^{emphasis mine}