204 lines
7.9 KiB
Plaintext
204 lines
7.9 KiB
Plaintext
[[search-aggregations-bucket-diversified-sampler-aggregation]]
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=== Diversified Sampler Aggregation
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Like the `sampler` aggregation this is a filtering aggregation used to limit any sub aggregations' processing to a sample of the top-scoring documents.
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The `diversified_sampler` aggregation adds the ability to limit the number of matches that share a common value such as an "author".
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NOTE: Any good market researcher will tell you that when working with samples of data it is important
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that the sample represents a healthy variety of opinions rather than being skewed by any single voice.
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The same is true with aggregations and sampling with these diversify settings can offer a way to remove the bias in your content (an over-populated geography,
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a large spike in a timeline or an over-active forum spammer).
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.Example use cases:
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* Tightening the focus of analytics to high-relevance matches rather than the potentially very long tail of low-quality matches
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* Removing bias from analytics by ensuring fair representation of content from different sources
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* Reducing the running cost of aggregations that can produce useful results using only samples e.g. `significant_terms`
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A choice of `field` or `script` setting is used to provide values used for de-duplication and the `max_docs_per_value` setting controls the maximum
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number of documents collected on any one shard which share a common value. The default setting for `max_docs_per_value` is 1.
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The aggregation will throw an error if the choice of `field` or `script` produces multiple values for a single document (de-duplication using multi-valued fields is not supported due to efficiency concerns).
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Example:
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We might want to see which tags are strongly associated with `#elasticsearch` on StackOverflow
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forum posts but ignoring the effects of some prolific users with a tendency to misspell #Kibana as #Cabana.
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[source,js]
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--------------------------------------------------
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POST /stackoverflow/_search?size=0
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{
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"query": {
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"query_string": {
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"query": "tags:elasticsearch"
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}
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},
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"aggs": {
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"my_unbiased_sample": {
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"diversified_sampler": {
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"shard_size": 200,
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"field" : "author"
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},
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"aggs": {
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"keywords": {
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"significant_terms": {
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"field": "tags",
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"exclude": ["elasticsearch"]
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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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// CONSOLE
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// TEST[setup:stackoverflow]
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Response:
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[source,js]
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--------------------------------------------------
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{
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...
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"aggregations": {
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"my_unbiased_sample": {
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"doc_count": 151,<1>
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"keywords": {<2>
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"doc_count": 151,
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"bg_count": 650,
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"buckets": [
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{
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"key": "kibana",
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"doc_count": 150,
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"score": 2.213,
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"bg_count": 200
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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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// TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,"hits": $body.hits,/]
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// TESTRESPONSE[s/2.213/$body.aggregations.my_unbiased_sample.keywords.buckets.0.score/]
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<1> 151 documents were sampled in total.
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<2> The results of the significant_terms aggregation are not skewed by any single author's quirks because we asked for a maximum of one post from any one author in our sample.
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==== Scripted example:
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In this scenario we might want to diversify on a combination of field values. We can use a `script` to produce a hash of the
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multiple values in a tags field to ensure we don't have a sample that consists of the same repeated combinations of tags.
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[source,js]
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--------------------------------------------------
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POST /stackoverflow/_search?size=0
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{
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"query": {
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"query_string": {
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"query": "tags:kibana"
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}
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},
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"aggs": {
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"my_unbiased_sample": {
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"diversified_sampler": {
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"shard_size": 200,
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"max_docs_per_value" : 3,
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"script" : {
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"lang": "painless",
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"source": "doc['tags'].hashCode()"
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}
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},
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"aggs": {
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"keywords": {
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"significant_terms": {
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"field": "tags",
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"exclude": ["kibana"]
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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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// CONSOLE
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// TEST[setup:stackoverflow]
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Response:
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[source,js]
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--------------------------------------------------
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{
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...
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"aggregations": {
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"my_unbiased_sample": {
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"doc_count": 6,
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"keywords": {
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"doc_count": 6,
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"bg_count": 650,
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"buckets": [
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{
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"key": "logstash",
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"doc_count": 3,
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"score": 2.213,
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"bg_count": 50
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},
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{
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"key": "elasticsearch",
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"doc_count": 3,
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"score": 1.34,
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"bg_count": 200
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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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// TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,"hits": $body.hits,/]
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// TESTRESPONSE[s/2.213/$body.aggregations.my_unbiased_sample.keywords.buckets.0.score/]
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// TESTRESPONSE[s/1.34/$body.aggregations.my_unbiased_sample.keywords.buckets.1.score/]
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==== shard_size
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The `shard_size` parameter limits how many top-scoring documents are collected in the sample processed on each shard.
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The default value is 100.
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==== max_docs_per_value
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The `max_docs_per_value` is an optional parameter and limits how many documents are permitted per choice of de-duplicating value.
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The default setting is "1".
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==== execution_hint
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The optional `execution_hint` setting can influence the management of the values used for de-duplication.
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Each option will hold up to `shard_size` values in memory while performing de-duplication but the type of value held can be controlled as follows:
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- hold field values directly (`map`)
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- hold ordinals of the field as determined by the Lucene index (`global_ordinals`)
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- hold hashes of the field values - with potential for hash collisions (`bytes_hash`)
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The default setting is to use `global_ordinals` if this information is available from the Lucene index and reverting to `map` if not.
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The `bytes_hash` setting may prove faster in some cases but introduces the possibility of false positives in de-duplication logic due to the possibility of hash collisions.
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Please note that Elasticsearch will ignore the choice of execution hint if it is not applicable and that there is no backward compatibility guarantee on these hints.
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==== Limitations
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[[div-sampler-breadth-first-nested-agg]]
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===== Cannot be nested under `breadth_first` aggregations
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Being a quality-based filter the diversified_sampler aggregation needs access to the relevance score produced for each document.
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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.
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In this situation an error will be thrown.
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===== Limited de-dup logic.
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The de-duplication logic applies only at a shard level so will not apply across shards.
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[[spec-syntax-geo-date-fields]]
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===== No specialized syntax for geo/date fields
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Currently the syntax for defining the diversifying values is defined by a choice of `field` or
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`script` - there is no added syntactical sugar for expressing geo or date units such as "7d" (7
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days). This support may be added in a later release and users will currently have to create these
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sorts of values using a script.
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