add a test case of incorrect merge sort

[sundy-li]

Related

fa3b003

Found in databendlabs/databend#1575

[jorgecarleitao]

found the root cause, fixing it

[jorgecarleitao]

This is fixed by #327

[jorgecarleitao]

Thanks a lot for taking the time to write a minimal test case; very useful! (also glad that you are using the merge-sort compute kernel; was not sure if it would ever be used ^_^)

[sundy-li]

Thanks a lot for taking the time to write a minimal test case; very useful! (also glad that you are using the merge-sort compute kernel; was not sure if it would ever be used ^_^)

Thanks for the fix. The merge-sort kernel is useful, and it also can be optimized by heap-sort when the limit option is very small (holding a priority queue in memory to reduce the compare times). I've created an issue in #329 to track it.