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Sampler aggregation

A filtering aggregation used to limit any sub aggregations' processing to a sample of the top-scoring documents.

  • Tightening the focus of analytics to high-relevance matches rather than the potentially very long tail of low-quality matches
  • Reducing the running cost of aggregations that can produce useful results using only samples e.g. significant_terms

Example:

A query on StackOverflow data for the popular term javascript OR the rarer term kibana will match many documents - most of them missing the word Kibana. To focus the significant_terms aggregation on top-scoring documents that are more likely to match the most interesting parts of our query we use a sample.

 POST /stackoverflow/_search?size=0 { "query": { "query_string": { "query": "tags:kibana OR tags:javascript" } }, "aggs": { "sample": { "sampler": { "shard_size": 200 }, "aggs": { "keywords": { "significant_terms": { "field": "tags", "exclude": [ "kibana", "javascript" ] } } } } } }

Response:

{ ... "aggregations": { "sample": { "doc_count": 200, "keywords": { "doc_count": 200, "bg_count": 650, "buckets": [ { "key": "elasticsearch", "doc_count": 150, "score": 1.078125, "bg_count": 200 }, { "key": "logstash", "doc_count": 50, "score": 0.5625, "bg_count": 50 } ] } } } }
  1. 200 documents were sampled in total. The cost of performing the nested significant_terms aggregation was therefore limited rather than unbounded.

Without the sampler aggregation the request query considers the full "long tail" of low-quality matches and therefore identifies less significant terms such as jquery and angular rather than focusing on the more insightful Kibana-related terms.

 POST /stackoverflow/_search?size=0 { "query": { "query_string": { "query": "tags:kibana OR tags:javascript" } }, "aggs": { "low_quality_keywords": { "significant_terms": { "field": "tags", "size": 3, "exclude": [ "kibana", "javascript" ] } } } }

Response:

{ ... "aggregations": { "low_quality_keywords": { "doc_count": 600, "bg_count": 650, "buckets": [ { "key": "angular", "doc_count": 200, "score": 0.02777, "bg_count": 200 }, { "key": "jquery", "doc_count": 200, "score": 0.02777, "bg_count": 200 }, { "key": "logstash", "doc_count": 50, "score": 0.0069, "bg_count": 50 } ] } } }

shard_size

The shard_size parameter limits how many top-scoring documents are collected in the sample processed on each shard. The default value is 100.

Limitations

Being a quality-based filter the sampler aggregation needs access to the relevance score produced for each document. It therefore cannot be nested under a terms aggregation which has the collect_mode switched from the default depth_first mode to breadth_first as this discards scores. In this situation an error will be thrown.