The join datatype is a special field that creates
parent/child relation within documents of the same index.
The relations section defines a set of possible relations within the documents,
each relation being a parent name and a child name.
A parent/child relation can be defined as follows:
PUT my_index
{
"mappings": {
"properties": {
"my_join_field": { 
"type": "join",
"relations": {
"question": "answer" 
}
}
}
}
}
| The name for the field |
|
Defines a single relation where |
To index a document with a join, the name of the relation and the optional parent
of the document must be provided in the source.
For instance the following example creates two parent documents in the question context:
PUT my_index/_doc/1?refresh
{
"text": "This is a question",
"my_join_field": {
"name": "question"
}
}
PUT my_index/_doc/2?refresh
{
"text": "This is another question",
"my_join_field": {
"name": "question"
}
}
|
This document is a |
When indexing parent documents, you can choose to specify just the name of the relation as a shortcut instead of encapsulating it in the normal object notation:
PUT my_index/_doc/1?refresh
{
"text": "This is a question",
"my_join_field": "question"
}
PUT my_index/_doc/2?refresh
{
"text": "This is another question",
"my_join_field": "question"
}
| Simpler notation for a parent document just uses the relation name. |
When indexing a child, the name of the relation as well as the parent id of the document
must be added in the _source.
It is required to index the lineage of a parent in the same shard so you must always route child documents using their greater parent id.
For instance the following example shows how to index two child documents:
PUT my_index/_doc/3?routing=1&refresh} } PUT my_index/_doc/4?routing=1&refresh { "text": "This is another answer", "my_join_field": { "name": "answer", "parent": "1" } }
| The routing value is mandatory because parent and child documents must be indexed on the same shard |
|
|
| The parent id of this child document |
The join field shouldn’t be used like joins in a relation database. In Elasticsearch the key to good performance
is to de-normalize your data into documents. Each join field, has_child or has_parent query adds a
significant tax to your query performance.
The only case where the join field makes sense is if your data contains a one-to-many relationship where one entity significantly outnumbers the other entity. An example of such case is a use case with products and offers for these products. In the case that offers significantly outnumbers the number of products then it makes sense to model the product as parent document and the offer as child document.
-
Only one
joinfield mapping is allowed per index. -
Parent and child documents must be indexed on the same shard.
This means that the same
routingvalue needs to be provided when getting, deleting, or updating a child document. - An element can have multiple children but only one parent.
-
It is possible to add a new relation to an existing
joinfield. - It is also possible to add a child to an existing element but only if the element is already a parent.
The parent-join creates one field to index the name of the relation
within the document (my_parent, my_child, …).
It also creates one field per parent/child relation.
The name of this field is the name of the join field followed by # and the
name of the parent in the relation.
So for instance for the my_parent ⇒ [my_child, another_child] relation,
the join field creates an additional field named my_join_field#my_parent.
This field contains the parent _id that the document links to
if the document is a child (my_child or another_child) and the _id of
document if it’s a parent (my_parent).
When searching an index that contains a join field, these two fields are always
returned in the search response:
GET my_index/_search
{
"query": {
"match_all": {}
},
"sort": ["_id"]
}Will return:
{
...,
"hits": {
"total" : {
"value": 4,
"relation": "eq"
},
"max_score": null,
"hits": [
{
"_index": "my_index",
"_type": "_doc",
"_id": "1",
"_score": null,
"_source": {
"text": "This is a question",
"my_join_field": "question"
},
"sort": [
"1"
]
},
{
"_index": "my_index",
"_type": "_doc",
"_id": "2",
"_score": null,
"_source": {
"text": "This is another question",
"my_join_field": "question"
},
"sort": [
"2"
]
},
{
"_index": "my_index",
"_type": "_doc",
"_id": "3",
"_score": null,
"_routing": "1",
"_source": {
"text": "This is an answer",
"my_join_field": {
"name": "answer",
"parent": "1" 
}
},
"sort": [
"3"
]
},
{
"_index": "my_index",
"_type": "_doc",
"_id": "4",
"_score": null,
"_routing": "1",
"_source": {
"text": "This is another answer",
"my_join_field": {
"name": "answer",
"parent": "1"
}
},
"sort": [
"4"
]
}
]
}
}
|
This document belongs to the |
|
This document belongs to the |
|
This document belongs to the |
| The linked parent id for the child document |
See the has_child and
has_parent queries,
the children aggregation,
and inner hits for more information.
The value of the join field is accessible in aggregations
and scripts, and may be queried with the
parent_id query:
GET my_index/_search
{
"query": {
"parent_id": {
"type": "answer",
"id": "1"
}
},
"aggs": {
"parents": {
"terms": {
"field": "my_join_field#question",
"size": 10
}
}
},
"script_fields": {
"parent": {
"script": {
"source": "doc['my_join_field#question']"
}
}
}
}
|
Querying the |
|
Aggregating on the |
| Accessing the parent id` field in scripts |
The join field 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 join field.
Global ordinals, by default, are built eagerly: if the index has changed,
global ordinals for the join field will be rebuilt as part of the refresh.
This can add significant time to the refresh. However most of the times this is the
right trade-off, otherwise global ordinals are rebuilt when the first parent-join
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 join
field may be attempt rebuilt within a single refresh interval when many writes
are occurring.
When the join field is used infrequently and writes occur frequently it may
make sense to disable eager loading:
PUT my_index
{
"mappings": {
"properties": {
"my_join_field": {
"type": "join",
"relations": {
"question": "answer"
},
"eager_global_ordinals": false
}
}
}
}The amount of heap used by global ordinals can be checked per parent relation as follows:
# Per-index GET _stats/fielddata?human&fields=my_join_field#question # Per-node per-index GET _nodes/stats/indices/fielddata?human&fields=my_join_field#question
It is also possible to define multiple children for a single parent:
PUT my_index
{
"mappings": {
"properties": {
"my_join_field": {
"type": "join",
"relations": {
"question": ["answer", "comment"]
}
}
}
}
}
|
|
Using multiple levels of relations to replicate a relational model is not recommended. Each level of relation adds an overhead at query time in terms of memory and computation. You should de-normalize your data if you care about performance.
Multiple levels of parent/child:
PUT my_index
{
"mappings": {
"properties": {
"my_join_field": {
"type": "join",
"relations": {
"question": ["answer", "comment"],
"answer": "vote"
}
}
}
}
}
|
|
|
|
The mapping above represents the following tree:
question
/ \
/ \
comment answer
|
|
voteIndexing a grandchild document requires a routing value equals
to the grand-parent (the greater parent of the lineage):
PUT my_index/_doc/3?routing=1&refresh
| This child document must be on the same shard than its grand-parent and parent |
|
The parent id of this document (must points to an |