Where clause pushdown through views, or worse CTEs. Views with window functions querying views with window functions. Queries with multiple windows. I didn't just randomly make the statement, modern query planners are an absolute arse to predict in the way that you can 100% of the time in external code. The moment you're doing anything really 'sequency' instead of just calculating some dumb running totals, window functions can become a nightmare, and I just made the statement so people don't automatically think there's a friction-free story for doing general sequence processing in SQL. Not sure why this has offended people so much.