87 lines
2.8 KiB
Plaintext
87 lines
2.8 KiB
Plaintext
[[query-dsl-top-children-query]]
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== Top Children Query
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deprecated[1.6.0, Use the `has_child` query instead]
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The `top_children` query runs the child query with an estimated hits
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size, and out of the hit docs, aggregates it into parent docs. If there
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aren't enough parent docs matching the requested from/size search
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request, then it is run again with a wider (more hits) search.
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The `top_children` also provide scoring capabilities, with the ability
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to specify `max`, `sum` or `avg` as the score type.
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One downside of using the `top_children` is that if there are more child
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docs matching the required hits when executing the child query, then the
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`total_hits` result of the search response will be incorrect.
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How many hits are asked for in the first child query run is controlled
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using the `factor` parameter (defaults to `5`). For example, when asking
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for 10 parent docs (with `from` set to 0), then the child query will
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execute with 50 hits expected. If not enough parents are found (in our
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example 10), and there are still more child docs to query, then the
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child search hits are expanded by multiplying by the
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`incremental_factor` (defaults to `2`).
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The required parameters are the `query` and `type` (the child type to
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execute the query on). Here is an example with all different parameters,
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including the default values:
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[source,js]
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--------------------------------------------------
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{
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"top_children" : {
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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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"score" : "max",
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"factor" : 5,
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"incremental_factor" : 2
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}
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}
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--------------------------------------------------
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[float]
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=== Scope
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A `_scope` can be defined on the query allowing to run aggregations on the
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same scope name that will work against the child documents. For example:
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[source,js]
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--------------------------------------------------
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{
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"top_children" : {
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"_scope" : "my_scope",
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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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[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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