Github user ChengXiangLi commented on the pull request:
https://github.com/apache/flink/pull/1377#issuecomment-157932844
@aljoscha , it's a very interesting topic, should we add a limitation that
only allow key fields to be no aggregated function field in `select` clause?
Oracle has this limitation while Mysql does not. Most of the time, select
non-aggregation field after groupby has no real meaning, while sometimes, if
user is aware that groupby some fielde would lead to other fields get grouped
as well, this feature would help to improve the performance and user
convenience, more described here:
http://dev.mysql.com/doc/refman/5.0/en/group-by-handling.html . Currently we
keep consistent with Mysql on this feature, allow no aggregation field after
`groupBy`, with no guarantee of its return value.
Actually, i prefer to the Mysql way, more flexible to user, what do you
think?
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