154 lines
6.3 KiB
Plaintext
154 lines
6.3 KiB
Plaintext
[[search-aggregations-bucket-diversified-sampler-aggregation]]
|
|
=== Sampler Aggregation
|
|
|
|
experimental[]
|
|
|
|
A filtering aggregation used to limit any sub aggregations' processing to a sample of the top-scoring documents. Diversity settings are
|
|
used to limit the number of matches that share a common value such as an "author".
|
|
|
|
.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`
|
|
|
|
|
|
Example:
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
{
|
|
"query": {
|
|
"match": {
|
|
"text": "iphone"
|
|
}
|
|
},
|
|
"aggs": {
|
|
"sample": {
|
|
"sampler": {
|
|
"shard_size": 200,
|
|
"field" : "user.id"
|
|
},
|
|
"aggs": {
|
|
"keywords": {
|
|
"significant_terms": {
|
|
"field": "text"
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
--------------------------------------------------
|
|
|
|
Response:
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
{
|
|
...
|
|
"aggregations": {
|
|
"sample": {
|
|
"doc_count": 1000,<1>
|
|
"keywords": {<2>
|
|
"doc_count": 1000,
|
|
"buckets": [
|
|
...
|
|
{
|
|
"key": "bend",
|
|
"doc_count": 58,
|
|
"score": 37.982536582524276,
|
|
"bg_count": 103
|
|
},
|
|
....
|
|
}
|
|
--------------------------------------------------
|
|
|
|
<1> 1000 documents were sampled in total becase we asked for a maximum of 200 from an index with 5 shards. The cost of performing the nested significant_terms aggregation was therefore limited rather than unbounded.
|
|
<2> The results of the significant_terms aggregation are not skewed by any single over-active Twitter user because we asked for a maximum of one tweet from any one user in our sample.
|
|
|
|
|
|
==== 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.
|
|
|
|
==== Controlling diversity
|
|
=`field` or `script` and `max_docs_per_value` settings are used to control the maximum number of documents collected on any one shard which share a common value.
|
|
The choice of value (e.g. `author`) is loaded from a regular `field` or derived dynamically by a `script`.
|
|
|
|
The aggregation will throw an error if the choice of field or script produces multiple values for a document.
|
|
It is currently not possible to offer this form of de-duplication using many values, primarily due to concerns over efficiency.
|
|
|
|
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).
|
|
|
|
==== Field
|
|
|
|
Controlling diversity using a field:
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
{
|
|
"aggs" : {
|
|
"sample" : {
|
|
"diverisfied_sampler" : {
|
|
"field" : "author",
|
|
"max_docs_per_value" : 3
|
|
}
|
|
}
|
|
}
|
|
}
|
|
--------------------------------------------------
|
|
|
|
Note that the `max_docs_per_value` setting applies on a per-shard basis only for the purposes of shard-local sampling.
|
|
It is not intended as a way of providing a global de-duplication feature on search results.
|
|
|
|
|
|
|
|
==== Script
|
|
|
|
Controlling diversity using a script:
|
|
|
|
[source,js]
|
|
--------------------------------------------------
|
|
{
|
|
"aggs" : {
|
|
"sample" : {
|
|
"diverisfied_sampler" : {
|
|
"script" : "doc['author'].value + '/' + doc['genre'].value"
|
|
}
|
|
}
|
|
}
|
|
}
|
|
--------------------------------------------------
|
|
Note in the above example we chose to use the default `max_docs_per_value` setting of 1 and combine author and genre fields to ensure
|
|
each shard sample has, at most, one match for an author/genre pair.
|
|
|
|
|
|
==== execution_hint
|
|
|
|
When using the settings to control diversity, 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
|
|
|
|
===== Cannot be nested under `breadth_first` aggregations
|
|
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.
|
|
|
|
===== Limited de-dup logic.
|
|
The de-duplication logic in the diversify settings applies only at a shard level so will not apply across shards.
|
|
|
|
===== 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 "1w" (1 week).
|
|
This support may be added in a later release and users will currently have to create these sorts of values using a script. |