275 lines
9.2 KiB
Plaintext
275 lines
9.2 KiB
Plaintext
[[search-aggregations-metrics-top-hits-aggregation]]
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=== Top hits Aggregation
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A `top_hits` metric aggregator keeps track of the most relevant document being aggregated. This aggregator is intended
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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.
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One or more bucket aggregators determines by which properties a result set get sliced into.
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==== Options
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* `from` - The offset from the first result you want to fetch.
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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.
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* `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>>
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* <<search-request-explain,Explain>>
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* <<search-request-named-queries-and-filters,Named filters and queries>>
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* <<search-request-source-filtering,Source filtering>>
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* <<search-request-script-fields,Script fields>>
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* <<search-request-fielddata-fields,Fielddata fields>>
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* <<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
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only the title field is being included in the source.
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[source,js]
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--------------------------------------------------
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{
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"aggs": {
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"top-tags": {
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"terms": {
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"field": "tags",
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"size": 3
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},
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"aggs": {
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"top_tag_hits": {
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"top_hits": {
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"sort": [
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{
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"last_activity_date": {
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"order": "desc"
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}
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}
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],
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"_source": {
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"include": [
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"title"
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]
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},
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"size" : 1
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}
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}
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}
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}
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}
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}
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--------------------------------------------------
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Possible response snippet:
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[source,js]
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--------------------------------------------------
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"aggregations": {
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"top-tags": {
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"buckets": [
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{
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"key": "windows-7",
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"doc_count": 25365,
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"top_tags_hits": {
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"hits": {
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"total": 25365,
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"max_score": 1,
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"hits": [
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{
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"_index": "stack",
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"_type": "question",
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"_id": "602679",
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"_score": 1,
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"_source": {
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"title": "Windows port opening"
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},
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"sort": [
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1370143231177
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]
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}
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]
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}
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}
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},
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{
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"key": "linux",
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"doc_count": 18342,
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"top_tags_hits": {
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"hits": {
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"total": 18342,
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"max_score": 1,
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"hits": [
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{
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"_index": "stack",
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"_type": "question",
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"_id": "602672",
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"_score": 1,
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"_source": {
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"title": "Ubuntu RFID Screensaver lock-unlock"
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},
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"sort": [
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1370143379747
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]
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}
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]
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}
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}
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},
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{
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"key": "windows",
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"doc_count": 18119,
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"top_tags_hits": {
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"hits": {
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"total": 18119,
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"max_score": 1,
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"hits": [
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{
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"_index": "stack",
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"_type": "question",
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"_id": "602678",
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"_score": 1,
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"_source": {
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"title": "If I change my computers date / time, what could be affected?"
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},
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"sort": [
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1370142868283
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]
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}
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]
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}
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}
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}
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]
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}
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}
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--------------------------------------------------
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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
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top documents. The ordering of the groups is determined by the relevancy of the first document in a group. In
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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
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belong to. By defining a `terms` aggregator on the `domain` field we group the result set of webpages by domain. The
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`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
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relevancy order of the most relevant document in a bucket.
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[source,js]
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--------------------------------------------------
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{
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"query": {
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"match": {
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"body": "elections"
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}
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},
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"aggs": {
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"top-sites": {
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"terms": {
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"field": "domain",
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"order": {
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"top_hit": "desc"
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}
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},
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"aggs": {
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"top_tags_hits": {
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"top_hits": {}
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},
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"top_hit" : {
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"max": {
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"script": "_score"
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}
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}
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}
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}
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}
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}
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--------------------------------------------------
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At the moment the `max` (or `min`) aggregator is needed to make sure the buckets from the `terms` aggregator are
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ordered according to the score of the most relevant webpage per domain. The `top_hits` aggregator isn't a metric aggregator
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and therefore can't be used in the `order` option of the `terms` aggregator.
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==== top_hits support in a nested or reverse_nested aggregator
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If the `top_hits` aggregator is wrapped in a `nested` or `reverse_nested` aggregator then nested hits are being returned.
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Nested hits are in a sense hidden mini documents that are part of regular document where in the mapping a nested field type
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has been configured. The `top_hits` aggregator has the ability to un-hide these documents if it is wrapped in a `nested`
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or `reverse_nested` aggregator. Read more about nested in the <<nested,nested type mapping>>.
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If nested type has been configured a single document is actually indexed as multiple Lucene documents and they share
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the same id. In order to determine the identity of a nested hit there is more needed than just the id, so that is why
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nested hits also include their nested identity. The nested identity is kept under the `_nested` field in the search hit
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and includes the array field and the offset in the array field the nested hit belongs to. The offset is zero based.
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Top hits response snippet with a nested hit, which resides in the third slot of array field `nested_field1` in document with id `1`:
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[source,js]
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--------------------------------------------------
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...
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"hits": {
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"total": 25365,
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"max_score": 1,
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"hits": [
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{
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"_index": "a",
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"_type": "b",
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"_id": "1",
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"_score": 1,
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"_nested" : {
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"field" : "nested_field1",
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"offset" : 2
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}
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"_source": ...
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},
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...
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]
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}
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...
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--------------------------------------------------
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If `_source` is requested then just the part of the source of the nested object is returned, not the entire source of the document.
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Also stored fields on the *nested* inner object level are accessible via `top_hits` aggregator residing in a `nested` or `reverse_nested` aggregator.
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Only nested hits will have a `_nested` field in the hit, non nested (regular) hits will not have a `_nested` field.
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The information in `_nested` can also be used to parse the original source somewhere else if `_source` isn't enabled.
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If there are multiple levels of nested object types defined in mappings then the `_nested` information can also be hierarchical
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in order to express the identity of nested hits that are two layers deep or more.
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In the example below a nested hit resides in the first slot of the field `nested_grand_child_field` which then resides in
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the second slow of the `nested_child_field` field:
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[source,js]
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--------------------------------------------------
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...
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"hits": {
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"total": 2565,
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"max_score": 1,
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"hits": [
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{
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"_index": "a",
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"_type": "b",
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"_id": "1",
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"_score": 1,
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"_nested" : {
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"field" : "nested_child_field",
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"offset" : 1,
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"_nested" : {
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"field" : "nested_grand_child_field",
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"offset" : 0
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}
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}
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"_source": ...
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},
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...
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]
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}
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...
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-------------------------------------------------- |