When are Aggregators and BufferAggregators used separately?

[lukoou3]

Differences between Aggregator and BufferAggregator ?
When are Aggregators and BufferAggregators used separately?
I want to implement AggregatorFactory，but I don't know when Druid uses Aggregator and BufferAggregator respectively, and how efficient is it.
When is AggregatorFactory.factorizeBuffered called?

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