101 lines
3.0 KiB
Plaintext
101 lines
3.0 KiB
Plaintext
[[query-dsl-has-child-query]]
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=== Has Child Query
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The `has_child` query works the same as the
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<<query-dsl-has-child-filter,has_child>> filter,
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by automatically wrapping the filter with a
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<<query-dsl-constant-score-query,constant_score>>
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(when using the default score type). It has the same syntax as the
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<<query-dsl-has-child-filter,has_child>> filter:
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[source,js]
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--------------------------------------------------
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{
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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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}
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}
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}
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}
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--------------------------------------------------
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An important difference with the `top_children` query is that this query
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is always executed in two iterations whereas the `top_children` query
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can be executed in one or more iteration. When using the `has_child`
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query the `total_hits` is always correct.
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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 types 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 type 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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[source,js]
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--------------------------------------------------
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{
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"has_child" : {
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"type" : "blog_tag",
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"score_mode" : "sum",
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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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[float]
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==== Min/Max Children
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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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[source,js]
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--------------------------------------------------
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{
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"has_child" : {
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"type" : "blog_tag",
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"score_mode" : "sum",
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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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<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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==== Memory Considerations
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In order to support parent-child joins, all of the (string) parent IDs
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must be resident in memory (in the <<index-modules-fielddata,field data cache>>.
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Additionally, every child document is mapped to its parent using a long
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value (approximately). It is advisable to keep the string parent ID short
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in order to reduce memory usage.
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You can check how much memory is being used by the ID cache using the
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<<indices-stats,indices stats>> or <<cluster-nodes-stats,nodes stats>>
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APIS, eg:
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[source,js]
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--------------------------------------------------
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curl -XGET "http://localhost:9200/_stats/id_cache?pretty&human"
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--------------------------------------------------
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