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_parent field
edit_parent
field
editA parent-child relationship can be established between documents in the same index by making one mapping type the parent of another:
PUT my_index { "mappings": { "my_parent": {}, "my_child": { "_parent": { "type": "my_parent" } } } } PUT my_index/my_parent/1 { "text": "This is a parent document" } PUT my_index/my_child/2?parent=1 { "text": "This is a child document" } PUT my_index/my_child/3?parent=1&refresh=true { "text": "This is another child document" } GET my_index/my_parent/_search { "query": { "has_child": { "type": "my_child", "query": { "match": { "text": "child document" } } } } }
The |
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Index a parent document. |
|
Index two child documents, specifying the parent document’s ID. |
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Find all parent documents that have children which match the query. |
See the has_child
and
has_parent
queries,
the children
aggregation,
and inner hits for more information.
The value of the _parent
field is accessible in aggregations
and scripts, and may be queried with the
parent_id
query:
GET my_index/_search { "query": { "parent_id": { "type": "my_child", "id": "1" } }, "aggs": { "parents": { "terms": { "field": "_parent", "size": 10 } } }, "script_fields": { "parent": { "script": { "inline": "doc['_parent']" } } } }
Querying the id of the |
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Aggregating on the |
|
Accessing the |
Parent-child restrictions
edit- The parent and child types must be different — parent-child relationships cannot be established between documents of the same type.
-
The
_parent.type
setting can only point to a type that doesn’t exist yet. This means that a type cannot become a parent type after it has been created. -
Parent and child documents must be indexed on the same shard. The
parent
ID is used as the routing value for the child, to ensure that the child is indexed on the same shard as the parent. This means that the sameparent
value needs to be provided when getting, deleting, or updating a child document.
Global ordinals
editParent-child uses global ordinals to speed up joins.
Global ordinals need to be rebuilt after any change to a shard. The more
parent id values are stored in a shard, the longer it takes to rebuild the
global ordinals for the _parent
field.
Global ordinals, by default, are built eagerly: if the index has changed,
global ordinals for the _parent
field will be rebuilt as part of the refresh.
This can add significant time the refresh. However most of the times this is the
right trade-off, otherwise global ordinals are rebuilt when the first parent-child
query or aggregation is used. This can introduce a significant latency spike for
your users and usually this is worse as multiple global ordinals for the _parent
field may be attempt rebuilt within a single refresh interval when many writes
are occurring.
When the parent/child is used infrequently and writes occur frequently it may make sense to disable eager loading:
PUT my_index { "mappings": { "my_parent": {}, "my_child": { "_parent": { "type": "my_parent", "eager_global_ordinals": false } } } }
The amount of heap used by global ordinals can be checked as follows:
# Per-index GET _stats/fielddata?human&fields=_parent # Per-node per-index GET _nodes/stats/indices/fielddata?human&fields=_parent