Sorting in a database makes sense even when you never use ORDER BY. You can do streaming joins/unions, and implement streaming, non-blocking aggregation operato

Sorting in a database makes sense even when you never use ORDER BY. You can do streaming joins/unions, and implement streaming, non-blocking aggregation operators (because you know when you've seen the last row for a grouping key, so you can output the result for that group immediately and evict its state from memory). Query plans will sometimes sort inputs (or intermediate results) for these reasons even when the final output is unordered; this is known as "interesting orders".