Fields API: reduce functions of GroupBuilder

com.twitter.scalding.GroupBuilder

mapStream

Full access to the Iterator of values. Avoid it if you can as this cannot be optimized much

scanLeft

A particular, but common, pattern to processes an iterator and return a new iterator.

foldLeft

Exactly like the above, but we only keep the last value.

reduce / mapReduceMap

Preferred operations, which are partially executed on the mappers.