ElasticSearch - Difference between Sub-aggregations and pipeline aggregations

Question

Could somebody explain me the difference between sub-aggregations and pipeline aggregations ?

For example: Which is the difference between sum metric aggregation inside a term bucket aggregation and a sum_buckets pipeline aggregation referenced to a term bucket aggregation?

Answer 1

Sub-aggregations operate on individual buckets of the previous level's aggregations, where as pipeline aggregations operate "after" "normal" aggregations. For example Derivative aggregation calculates the derivate of the aggregated value.

Basically sub-aggregations are done in the context of an aggregations' bucket, but pipeline aggregations are calculated across buckets (after their values have been computed).