291 lines
7.8 KiB
Plaintext
291 lines
7.8 KiB
Plaintext
[[search-facets-terms-facet]]
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=== Terms Facet
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Allow to specify field facets that return the N most frequent terms. For
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example:
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"tag" : {
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"terms" : {
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"field" : "tag",
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"size" : 10
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}
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}
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}
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}
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--------------------------------------------------
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It is preferred to have the terms facet executed on a non analyzed
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field, or a field without a large number of terms it breaks to.
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==== Accuracy Control
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added[0.90.6]
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The `size` parameter defines how many top terms should be returned out
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of the overall terms list. By default, the node coordinating the
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search process will ask each shard to provide its own top `size` terms
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and once all shards respond, it will reduces the results to the final list
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that will then be sent back to the client. This means that if the number
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of unique terms is greater than `size`, the returned list is slightly off
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and not accurate (it could be that the term counts are slightly off and it
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could even be that a term that should have been in the top `size` entries
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was not returned).
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The higher the requested `size` is, the more accurate the results will be,
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but also, the more expensive it will be to compute the final results (both
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due to bigger priority queues that are managed on a shard level and due to
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bigger data transfers between the nodes and the client). In an attempt to
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minimize the extra work that comes with bigger requested `size` the
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`shard_size` parameter was introduced. When defined, it will determine
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how many terms the coordinating node will request from each shard. Once
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all the shards responded, the coordinating node will then reduce them
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to a final result which will be based on the `size` parameter - this way,
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one can increase the accuracy of the returned terms and avoid the overhead
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of streaming a big list of terms back to the client.
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Note that `shard_size` cannot be smaller than `size`... if that's the case
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elasticsearch will override it and reset it to be equal to `size`.
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==== Ordering
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Allow to control the ordering of the terms facets, to be ordered by
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`count`, `term`, `reverse_count` or `reverse_term`. The default is
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`count`. Here is an example:
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"tag" : {
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"terms" : {
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"field" : "tag",
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"size" : 10,
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"order" : "term"
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}
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}
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}
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}
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--------------------------------------------------
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==== All Terms
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Allow to get all the terms in the terms facet, ones that do not match a
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hit, will have a count of 0. Note, this should not be used with fields
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that have many terms.
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"tag" : {
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"terms" : {
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"field" : "tag",
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"all_terms" : true
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}
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}
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}
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}
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--------------------------------------------------
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==== Excluding Terms
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It is possible to specify a set of terms that should be excluded from
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the terms facet request result:
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"tag" : {
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"terms" : {
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"field" : "tag",
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"exclude" : ["term1", "term2"]
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}
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}
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}
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}
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--------------------------------------------------
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==== Regex Patterns
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The terms API allows to define regex expression that will control which
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terms will be included in the faceted list, here is an example:
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"tag" : {
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"terms" : {
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"field" : "tag",
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"regex" : "_regex expression here_",
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"regex_flags" : "DOTALL"
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}
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}
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}
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}
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--------------------------------------------------
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Check
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http://download.oracle.com/javase/6/docs/api/java/util/regex/Pattern.html#field_summary[Java
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Pattern API] for more details about `regex_flags` options.
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==== Term Scripts
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Allow to define a script for terms facet to process the actual term that
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will be used in the term facet collection, and also optionally control
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its inclusion or not.
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The script can either return a boolean value, with `true` to include it
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in the facet collection, and `false` to exclude it from the facet
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collection.
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Another option is for the script to return a `string` controlling the
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term that will be used to count against. The script execution will
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include the term variable which is the current field term used.
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For example:
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"tag" : {
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"terms" : {
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"field" : "tag",
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"size" : 10,
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"script" : "term + 'aaa'"
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}
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}
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}
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}
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--------------------------------------------------
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And using the boolean feature:
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"tag" : {
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"terms" : {
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"field" : "tag",
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"size" : 10,
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"script" : "term == 'aaa' ? true : false"
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}
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}
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}
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}
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--------------------------------------------------
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==== Multi Fields
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The term facet can be executed against more than one field, returning
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the aggregation result across those fields. For example:
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"tag" : {
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"terms" : {
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"fields" : ["tag1", "tag2"],
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"size" : 10
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}
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}
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}
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}
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--------------------------------------------------
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==== Script Field
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A script that provides the actual terms that will be processed for a
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given doc. A `script_field` (or `script` which will be used when no
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`field` or `fields` are provided) can be set to provide it.
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As an example, a search request (that is quite "heavy") can be executed
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and use either `_source` itself or `_fields` (for stored fields) without
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needing to load the terms to memory (at the expense of much slower
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execution of the search, and causing more IO load):
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"my_facet" : {
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"terms" : {
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"script_field" : "_source.my_field",
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"size" : 10
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}
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}
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}
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}
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--------------------------------------------------
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Or:
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[source,js]
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--------------------------------------------------
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{
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"query" : {
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"match_all" : { }
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},
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"facets" : {
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"my_facet" : {
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"terms" : {
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"script_field" : "_fields['my_field']",
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"size" : 10
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}
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}
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}
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}
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--------------------------------------------------
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Note also, that the above will use the whole field value as a single
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term.
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==== _index
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The term facet allows to specify a special field name called `_index`.
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This will return a facet count of hits per `_index` the search was
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executed on (relevant when a search request spans more than one index).
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==== Memory Considerations
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Term facet causes the relevant field values to be loaded into memory.
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This means that per shard, there should be enough memory to contain
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them. It is advisable to explicitly set the fields to be `not_analyzed`
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or make sure the number of unique tokens a field can have is not large.
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