Property of a stream of values that reflects whether those values consistently increase (or repeat) or decrease (or repeat), but in either case never change direction, that is, from ascending to descending or vice versa. s-Server streams generally need to increase monotonically by ROWTIME. Some operators require monotonically increasing values; in particular, the streaming GROUP BY operator requires that at least one of its arguments be monotonic and non-constant. Also, if SQLstream can deduce that an expression is monotonically increasing, it may be able to evaluate the the query more efficiently. Using the MONOTONIC function asserts to SQLstream that x is monotonic.