Improve aggregation code readability

### Is your feature request related to a problem or challenge?

I'm reading aggregation code recently, found it's hard to read due to different `AggregateMode`, these modes are mixed up in the code.

### Describe the solution you'd like

Might split stream based on `AggregateMode`?

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

No file, test, or entry point is identified in the issue. Start by locating the aggregation code and references to `AggregateMode`, then determine how the modes are currently mixed and what stream split is intended; done means the aggregation code is more readable without changing its behavior.

Written by the indexing model from the issue text.