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Add a warning about the impact of sorting terms aggregations on the accuracy of doc counts.

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Adrien Grand 2016-04-07 10:37:26 +02:00
parent f9d1e8a5f3
commit 1d0239c125
1 changed files with 9 additions and 0 deletions
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docs/reference/aggregations/bucket/terms-aggregation.asciidoc
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@ -314,6 +314,15 @@ Ordering the buckets by multi value metrics sub-aggregation (identified by the a
}
--------------------------------------------------
WARNING: Sorting by ascending `_count` or by sub aggregation is discouraged as it increases the
<<search-aggregations-bucket-terms-aggregation-approximate-counts,error>> on document counts.
It is fine when a single shard is queried, or when the field that is being aggregated was used
as a routing key at index time: in these cases results will be accurate since shards have disjoint
values. However otherwise, errors are unbounded. One particular case that could still be useful
is sorting by <<search-aggregations-metrics-min-aggregation,`min`>> or
<<search-aggregations-metrics-max-aggregation,`max`>> aggregation: counts will not be accurate
but at least the top buckets will be correctly picked.
It is also possible to order the buckets based on a "deeper" aggregation in the hierarchy. This is supported as long
as the aggregations path are of a single-bucket type, where the last aggregation in the path may either be a single-bucket
one or a metrics one. If it's a single-bucket type, the order will be defined by the number of docs in the bucket (i.e. `doc_count`),