166 lines
4.5 KiB
Plaintext
166 lines
4.5 KiB
Plaintext
[[mapping-parent-field]]
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=== `_parent` field
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added[2.0.0,The parent-child implementation has been completely rewritten. It is advisable to reindex any 1.x indices which use parent-child to take advantage of the new optimizations]
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A parent-child relationship can be established between documents in the same
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index by making one mapping type the parent of another:
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[source,js]
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--------------------------------------------------
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PUT my_index
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{
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"mappings": {
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"my_parent": {},
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"my_child": {
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"_parent": {
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"type": "my_parent" <1>
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}
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}
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}
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}
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PUT my_index/my_parent/1 <2>
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{
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"text": "This is a parent document"
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}
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PUT my_index/my_child/2?parent=1 <3>
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{
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"text": "This is a child document"
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}
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PUT my_index/my_child/3?parent=1 <3>
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{
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"text": "This is another child document"
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}
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GET my_index/my_parent/_search
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{
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"query": {
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"has_child": { <4>
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"type": "my_child",
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"query": {
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"match": {
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"text": "child document"
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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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// AUTOSENSE
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<1> The `my_parent` type is parent to the `my_child` type.
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<2> Index a parent document.
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<3> Index two child documents, specifying the parent document's ID.
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<4> Find all parent documents that have children which match the query.
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See the <<query-dsl-has-child-query,`has_child`>> and
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<<query-dsl-has-parent-query,`has_parent`>> queries,
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the <<search-aggregations-bucket-children-aggregation,`children`>> aggregation,
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and <<parent-child-inner-hits,inner hits>> for more information.
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The value of the `_parent` field is accessible in queries, aggregations, scripts,
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and when sorting:
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[source,js]
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--------------------------
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GET my_index/_search
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{
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"query": {
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"terms": {
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"_parent": [ "1" ] <1>
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}
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},
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"aggs": {
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"parents": {
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"terms": {
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"field": "_parent", <2>
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"size": 10
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}
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}
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},
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"sort": [
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{
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"_parent": { <3>
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"order": "desc"
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}
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}
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],
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"script_fields": {
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"parent": {
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"script": "doc['_parent']" <4>
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}
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}
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}
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--------------------------
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// AUTOSENSE
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<1> Querying on the `_parent` field (also see the <<query-dsl-has-parent-query,`has_parent` query>> and the <<query-dsl-has-child-query,`has_child` query>>)
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<2> Aggregating on the `_parent` field (also see the <<search-aggregations-bucket-children-aggregation,`children`>> aggregation)
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<3> Sorting on the `_parent` field
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<4> Accessing the `_parent` field in scripts (inline scripts must be <<enable-dynamic-scripting,enabled>> for this example to work)
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==== Parent-child restrictions
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* The parent and child types must be different -- parent-child relationships
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cannot be established between documents of the same type.
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* The `_parent.type` setting can only point to a type that doesn't exist yet.
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This means that a type cannot become a parent type after it is has been
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created.
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* Parent and child documents must be indexed on the same shard. The `parent`
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ID is used as the <<mapping-routing-field,routing>> value for the child,
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to ensure that the child is indexed on the same shard as the parent.
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This means that the same `parent` value needs to be provided when
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<<docs-get,getting>>, <<docs-delete,deleting>>, or <<docs-update,updating>>
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a child document.
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==== Global ordinals
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Parent-child uses <<global-ordinals,global ordinals>> to speed up joins.
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Global ordinals need to be rebuilt after any change to a shard. The more
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parent id values are stored in a shard, the longer it takes to rebuild the
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global ordinals for the `_parent` field.
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Global ordinals, by default, are built lazily: the first parent-child query or
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aggregation after a refresh will trigger building of global ordinals. This can
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introduce a significant latency spike for your users. You can use
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<<fielddata-loading,eager_global_ordinals>> to shift the cost of building global
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ordinals from query time to refresh time, by mapping the `_parent` field as follows:
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[source,js]
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--------------------------------------------------
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PUT my_index
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{
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"mappings": {
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"my_parent": {},
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"my_child": {
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"_parent": {
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"type": "my_parent",
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"fielddata": {
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"loading": "eager_global_ordinals"
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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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// AUTOSENSE
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The amount of heap used by global ordinals can be checked as follows:
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[source,sh]
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--------------------------------------------------
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# Per-index
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GET _stats/fielddata?human&fields=_parent
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# Per-node per-index
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GET _nodes/stats/indices/fielddata?human&fields=_parent
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--------------------------------------------------
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// AUTOSENSE
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