there is already groupArray, there is also arraySort, merging of two sorted arrays is O(n+m) operation.
Is it possible to implement groupArraySorted? And also with limit? With limit it should be efficient even on very big datasets.

It could be used to solve 'take fist/last N elements in each group' / lastpoint -alike tasks in selects and incrementally in materialized view (with AggregatingMergeTree).