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