[DOCS] Rewrite `has_child` query to use new format (#44190)
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James Rodewig
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[[query-dsl-has-child-query]]
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=== Has Child Query
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The `has_child` filter accepts a query and the child type to run against, and
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results in parent documents that have child docs matching the query. Here is
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an example:
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Returns parent documents whose <<parent-join,joined>> child documents match a
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provided query. You can create parent-child relationships between documents in
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the same index using a <<parent-join,join>> field mapping.
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[WARNING]
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====
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Because it performs a join, the `has_child` is slow compared to other queries.
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Its performance degrades as the number of matching child documents pointing to
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unique parent documents increases. Each `has_child` query in a search can
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increase query time significantly.
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If you care about query performance, do not use this query. If you need to use
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the `has_child` query, use it as rarely as possible.
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====
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[[has-child-query-ex-request]]
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==== Example request
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[[has-child-index-setup]]
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===== Index setup
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To use the `has_child` query, your index must include a <<parent-join,join>>
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field mapping. For example:
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[source,js]
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--------------------------------------------------
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GET /_search
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----
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PUT /my_index
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{
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"query": {
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"has_child" : {
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"type" : "blog_tag",
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"query" : {
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"term" : {
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"tag" : "something"
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"mappings": {
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"properties" : {
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"my-join-field" : {
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"type" : "join",
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"relations": {
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"parent": "child"
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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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// TESTSETUP
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Note that the `has_child` is a slow query compared to other queries in the
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query dsl due to the fact that it performs a join. The performance degrades
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as the number of matching child documents pointing to unique parent documents
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increases. If you care about query performance you should not use this query.
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However if you do happen to use this query then use it as little as possible.
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Each `has_child` query that gets added to a search request can increase query
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time significantly.
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[float]
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==== Scoring capabilities
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The `has_child` also has scoring support. The
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supported score modes are `min`, `max`, `sum`, `avg` or `none`. The default is
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`none` and yields the same behaviour as in previous versions. If the
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score mode is set to another value than `none`, the scores of all the
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matching child documents are aggregated into the associated parent
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documents. The score type can be specified with the `score_mode` field
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inside the `has_child` query:
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[[has-child-query-ex-query]]
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===== Example query
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[source,js]
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--------------------------------------------------
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----
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GET /_search
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{
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"query": {
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"has_child" : {
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"type" : "blog_tag",
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"score_mode" : "min",
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"type" : "child",
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"query" : {
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"term" : {
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"tag" : "something"
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}
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}
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"match_all" : {}
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},
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"max_children": 10,
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"min_children": 2,
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"score_mode" : "min"
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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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[float]
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[[min-max-children]]
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==== Min/Max Children
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[[has-child-top-level-params]]
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==== Top-level parameters for `has_child`
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The `has_child` query allows you to specify that a minimum and/or maximum
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number of children are required to match for the parent doc to be considered
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a match:
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`type`::
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(string) Required. Name of the child relationship mapped for the
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<<parent-join,join>> field.
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`query`::
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(query object) Required. Query you wish to run on child documents of the `type`
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field. If a child document matches the search, the query returns the parent
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document.
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`ignore_unmapped`::
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+
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--
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(boolean) Optional. Indicates whether to ignore an unmapped `type` and not return
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any documents instead of an error. Defaults to `false`.
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If `false`, {es} returns an error if the `type` is unmapped.
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You can use this parameter to query multiple indices that may not contain the
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`type`.
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--
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`max_children`::
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(integer) Optional. Maximum number of child documents that match the `query`
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allowed for a returned parent document. If the parent document exceeds this
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limit, it is excluded from the search results.
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`min_children`::
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(integer) Optional. Minimum number of child documents that match the `query`
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required to match the query for a returned parent document. If the parent
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document does not meet this limit, it is excluded from the search results.
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`score_mode`::
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+
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--
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(string) Optional. Indicates how scores for matching child documents affect the
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root parent document's <<query-filter-context,relevance score>>. Valid values
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are:
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`none` (Default)::
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Do not use the relevance scores of matching child documents. The query assigns
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parent documents a score of `0`.
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`avg`::
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Use the mean relevance score of all matching child documents.
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`max`::
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Uses the highest relevance score of all matching child documents.
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`min`::
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Uses the lowest relevance score of all matching child documents.
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`sum`::
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Add together the relevance scores of all matching child documents.
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--
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[[has-child-query-notes]]
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==== Notes
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[[has-child-query-performance]]
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===== Sorting
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You cannot sort the results of a `has_child` query using standard
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<<search-request-sort,sort options>>.
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If you need to sort returned documents by a field in their child documents, use
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a `function_score` query and sort by `_score`. For example, the following query
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sorts returned documents by the `click_count` field of their child documents.
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[source,js]
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--------------------------------------------------
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----
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GET /_search
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{
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"query": {
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"has_child" : {
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"type" : "blog_tag",
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"score_mode" : "min",
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"min_children": 2, <1>
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"max_children": 10, <1>
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"query" : {
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"term" : {
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"tag" : "something"
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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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<1> Both `min_children` and `max_children` are optional.
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The `min_children` and `max_children` parameters can be combined with
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the `score_mode` parameter.
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[float]
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==== Ignore Unmapped
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When set to `true` the `ignore_unmapped` option will ignore an unmapped `type`
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and will not match any documents for this query. This can be useful when
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querying multiple indexes which might have different mappings. When set to
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`false` (the default value) the query will throw an exception if the `type`
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is not mapped.
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[float]
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==== Sorting
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Parent documents can't be sorted by fields in matching child documents via the
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regular sort options. If you need to sort parent document by field in the child
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documents then you should use the `function_score` query and then just sort
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by `_score`.
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Sorting blogs by child documents' `click_count` field:
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[source,js]
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--------------------------------------------------
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GET /_search
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{
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"query": {
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"has_child" : {
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"type" : "blog_tag",
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"score_mode" : "max",
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"type" : "child",
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"query" : {
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"function_score" : {
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"script_score": {
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"script": "_score * doc['click_count'].value"
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}
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}
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}
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},
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"score_mode" : "max"
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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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