iotdb diff 是计算差值，但是不能计算每个设备的差值

diff 计算一个设备的差值没问题， 要按每个设备分组，然后计算差值就不行。有别的方式吗？ 或者后面会不会支持， 表模式下面

Start by reproducing the reported diff behavior for one device and then for multiple devices grouped by device in table mode. Review the current diff support and any documented alternatives; done should mean that per-device grouped differences are either supported or the available limitation and workaround are clearly documented.

Written by the indexing model from the issue text.