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top_hits aggregation that keeps track of the most relevant do…
…cument being aggregated per bucket. Closes #6124
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docs/reference/search/aggregations/bucket/tophits-aggregation.asciidoc
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| [[search-aggregations-bucket-top-hits-aggregation]] | ||
| === Top hits Aggregation | ||
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| coming[1.3.0] | ||
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| The `top_hits` aggregator keeps track of the most relevant document being aggregated. This aggregator is intended to be | ||
| used as a sub aggregator, so that the top matching documents can be aggregated per bucket. | ||
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| The `top_hits` aggregator can effectively be used to group result sets by certain fields via a bucket aggregator. | ||
| One or more bucket aggregators determines by which properties a result set get sliced into. | ||
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| This aggregator can't hold any sub-aggregators and therefor can only be used as a leaf aggregator. | ||
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| ==== Options | ||
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| * `size` - The maximum number of top matching hits to return per bucket. By default the top three matching hits are returned. | ||
| * `sort` - How the top matching hits should be sorted. By default the hits are sorted by the score of the main query. | ||
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| ==== Supported per hit features | ||
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| The top_hits aggregation returns regular search hits, because of this many per hit features can be supported: | ||
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| * <<search-request-highlighting,Highlighting>> | ||
| * <<search-request-explain,Explain>> | ||
| * <<search-request-named-queries-and-filters,Named filters and queries>> | ||
| * <<search-request-source-filtering,Source filtering>> | ||
| * <<search-request-script-fields,Script fields>> | ||
| * <<search-request-fielddata-fields,Fielddata fields>> | ||
| * <<search-request-version,Include versions>> | ||
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| ==== Example | ||
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| In the following example we group the questions by tag and per tag we show the last active question. For each question | ||
| only the title field is being included in the source. | ||
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| [source,js] | ||
| -------------------------------------------------- | ||
| { | ||
| "aggs": { | ||
| "terms": { | ||
| "top-tags": { | ||
| "field": "tags", | ||
| "size": 3 | ||
| }, | ||
| "aggs": { | ||
| "top_tag_hits": { | ||
| "top_hits": { | ||
| "sort": [ | ||
| { | ||
| "last_activity_date": { | ||
| "order": "desc" | ||
| } | ||
| } | ||
| ], | ||
| "_source": { | ||
| "include": [ | ||
| "title" | ||
| ] | ||
| }, | ||
| "size" : 1 | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| -------------------------------------------------- | ||
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| Possible response snippet: | ||
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| [source,js] | ||
| -------------------------------------------------- | ||
| "aggregations": { | ||
| "top-tags": { | ||
| "buckets": [ | ||
| { | ||
| "key": "windows-7", | ||
| "doc_count": 25365, | ||
| "top_tags_hits": { | ||
| "hits": { | ||
| "total": 25365, | ||
| "max_score": 1, | ||
| "hits": [ | ||
| { | ||
| "_index": "stack", | ||
| "_type": "question", | ||
| "_id": "602679", | ||
| "_score": 1, | ||
| "_source": { | ||
| "title": "Windows port opening" | ||
| }, | ||
| "sort": [ | ||
| 1370143231177 | ||
| ] | ||
| } | ||
| ] | ||
| } | ||
| } | ||
| }, | ||
| { | ||
| "key": "linux", | ||
| "doc_count": 18342, | ||
| "top_tags_hits": { | ||
| "hits": { | ||
| "total": 18342, | ||
| "max_score": 1, | ||
| "hits": [ | ||
| { | ||
| "_index": "stack", | ||
| "_type": "question", | ||
| "_id": "602672", | ||
| "_score": 1, | ||
| "_source": { | ||
| "title": "Ubuntu RFID Screensaver lock-unlock" | ||
| }, | ||
| "sort": [ | ||
| 1370143379747 | ||
| ] | ||
| } | ||
| ] | ||
| } | ||
| } | ||
| }, | ||
| { | ||
| "key": "windows", | ||
| "doc_count": 18119, | ||
| "top_tags_hits": { | ||
| "hits": { | ||
| "total": 18119, | ||
| "max_score": 1, | ||
| "hits": [ | ||
| { | ||
| "_index": "stack", | ||
| "_type": "question", | ||
| "_id": "602678", | ||
| "_score": 1, | ||
| "_source": { | ||
| "title": "If I change my computers date / time, what could be affected?" | ||
| }, | ||
| "sort": [ | ||
| 1370142868283 | ||
| ] | ||
| } | ||
| ] | ||
| } | ||
| } | ||
| } | ||
| ] | ||
| } | ||
| } | ||
| -------------------------------------------------- | ||
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| ==== Field collapse example | ||
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| Field collapsing or result grouping is a feature that logically groups a result set into groups and per group returns | ||
| top documents. The ordering of the groups is determined by the relevancy of the first document in a group. In | ||
| Elasticsearch this can be implemented via a bucket aggregator that wraps a `top_hits` aggregator as sub-aggregator. | ||
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| In the example below we search across crawled webpages. For each webpage we store the body and the domain the webpage | ||
| belong to. By defining a `terms` aggregator on the `domain` field we group the result set of webpages by domain. The | ||
| `top_docs` aggregator is then defined as sub-aggregator, so that the top matching hits are collected per bucket. | ||
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| Also a `max` aggregator is defined which is used by the `terms` aggregator's order feature the return the buckets by | ||
| relevancy order of the most relevant document in a bucket. | ||
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| [source,js] | ||
| -------------------------------------------------- | ||
| { | ||
| "query": { | ||
| "match": { | ||
| "body": "elections" | ||
| } | ||
| }, | ||
| "aggs": { | ||
| "top-sites": { | ||
| "terms": { | ||
| "field": "domain", | ||
| "order": { | ||
| "top_hit": "desc" | ||
| } | ||
| }, | ||
| "aggs": { | ||
| "top_tags_hits": { | ||
| "top_hits": {} | ||
| }, | ||
| "top_hit" : { | ||
| "max": { | ||
| "script": "_doc.score" | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| } | ||
| -------------------------------------------------- | ||
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| At the moment the `max` (or `min`) aggregator is needed to make sure the buckets from the `terms` aggregator are | ||
| ordered according to the score of the most relevant webpage per domain. The `top_hits` aggregator isn't a metric aggregator | ||
| and therefor can't be used in the `order` option of the `terms` aggregator. |
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