Hi DuyHai, thanks for your response.
I understand the reservations about implementing sorting in Cassandra. But I think it is analogous to filtering. It may be bad in the general case, but can be useful for particular use cases. If Cassandra does not provide “order-by”, then the ordering has to be done in the client (or an intermediate tool like Spark). The cost of ordering will be the same, but in the Top N use case, far more data has to be transferred to the client when the client has to do the sorting. So I think, with a qualification “ALLOW ORDERING”, it would be reasonable to support “order by” on aggregated values. Thanks… Roger From: DuyHai Doan [mailto:doanduy...@gmail.com] Sent: Tuesday, June 06, 2017 12:31 AM To: Roger Fischer (CW) <rfis...@brocade.com> Cc: user@cassandra.apache.org Subject: Re: Order by for aggregated values First Group By is only allowed on partition keys and clustering columns, not on arbitrary column. The internal implementation of group by tries to fetch data on clustering order to avoid having to "re-sort" them in memory which would be very expensive Second, group by works best when restricted to a single partition other wise it will force Cassandra to do a range scan so poor performance For all of those reasons I don't expect an "order by" on aggregated values to be available any soon Furthermore, Cassandra is optimised for real-time transactional scenarios, the group by/order by/limit is typically a classical analytics scenario, I would recommend to use the appropriate tool like Spark for that Le 6 juin 2017 04:00, "Roger Fischer (CW)" <rfis...@brocade.com<mailto:rfis...@brocade.com>> a écrit : Hello, is there any intent to support “order by” and “limit” on aggregated values? For time series data, top n queries are quite common. Group-by was the first step towards supporting such queries, but ordering by value and limiting the results are also required. Thanks… Roger
