We recently upgraded to Elasticsearch 5.6.9 from 5.2.2, with plans for for upgrading to 6.x in the works although requires more work. Through our initial usage the return json was identical to 5.2.2 and so the object we used internally represent it did not change.

Then during testing another use case for elasticsearch it appears that another field called "num_reduce_phases" started showing up in the response json from elasticsearch. From looking at the code it appears that in version 5.4 this is returned if the number of the reduce phases exceeds 1.

Could some one help me understand what might cause elasticsearch reduce phases to be greater than one? Does it have to do with usage of functions in the query? Size of the data being searched? The usage of field specific queries?


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