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Added section for reducer aggregations in the main aggregation docs page

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Colin Goodheart-Smithe 2015-04-15 14:23:29 +01:00
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@ -116,6 +116,38 @@ aggregated for the buckets created by their "parent" bucket aggregation.
There are different bucket aggregators, each with a different "bucketing" strategy. Some define a single bucket, some
define fixed number of multiple buckets, and others dynamically create the buckets during the aggregation process.
[float]
=== Reducer Aggregations
coming[2.0.0]
experimental[]
Reducer aggregations work on the outputs produced from other aggregations rather than from document sets, adding
information to the output tree. There are many different types of reducer, each computing different information from
other aggregations, but these type can broken down into two families:
_Parent_::
A family of reducer aggregations that is provided with the output of its parent aggregation and is able
to compute new buckets or new aggregations to add to existing buckets.
_Sibling_::
Reducer aggregations that are provided with the output of a sibling aggregation and are able to compute a
new aggregation which will be at the same level as the sibling aggregation.
Reducer aggregations can reference the aggregations they need to perform their computation by using the `buckets_paths`
parameter to indicate the paths to the required metrics. The syntax for defining these paths can be found in the
<<search-aggregations-bucket-terms-aggregation-order, terms aggregation order>> section.
?????? SHOULD THE SECTION ABOUT DEFINING AGGREGATION PATHS
BE IN THIS PAGE AND REFERENCED FROM THE TERMS AGGREGATION DOCUMENTATION ???????
Reducer aggregations cannot have sub-aggregations but depending on the type it can reference another reducer in the `buckets_path`
allowing reducers to be chained.
NOTE: Because reducer aggregations only add to the output, when chaining reducer aggregations the output of each reducer will be
included in the final output.
[float]
=== Caching heavy aggregations