The monotonic function can be useful, but it is not documented, and is not guaranteed to return sequential numbers with no gaps starting from 1. That's not promised by the definition of monotonic: "varying in such a way that it either never decreases or never increases". In his paper on adding sequence numbers, http://support.sas.com/resources/papers/proceedings14/1277-2014.pdf , Howard Schreier says: "The catch is that the MONOTONIC function is not documented. Organizations may have policies prohibiting the use of undocumented features. Even in the absence of such a policy, a coder may be reluctant to rely on the function. In developing examples to illustrate the usage of the MONOTONIC function, the author encountered some surprising and disconcerting results. In some cases, series returned by MONOTONIC began not with 1 (one), but rather with higher integers. Unfortunately, it seemed not possible to produce such results consistently, or to experimentally determine just what circumstances cause the surprising and suspicious offsets."
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