Elasticsearch Nested-field vs Depth? Check for document depth via Kibana? - elasticsearch

Question

I'm reading about mapping in elasticsearch and I see these 2 terms: Nested-field & Depth. I think these 2 terms are quite equivalent. I'm currently confused by these 2. Please can anyone clear me out? Thank you.
And btw, are there any ways to check a document depth via Kibana?
Sorry for my english.

Answer 1

The source of confusion is probably because in Elasticsearch term nested can be used in two different contexts:
"nested" as a regular JSON notation nested, i.e. JSON object within JSON object;
"nested" as Elasticsearch nested data type.
In the mappings documentation page when they mention "depth" they refer to the first meaning. Here the setting index.mapping.depth.limit defines how deeply nested can your JSON documents be.
How is JSON depth interpreted by Elasticsearch mapping?
Here is an example of JSON document with depth 1:
{
"name": "John",
"age": 30
}
Now with depth 2:
{
"name": "John",
"age": 30,
"cars": {
"car1": "Ford",
"car2": "BMW",
"car3": "Fiat"
}
}
By default (as of ES 6.3) the depth cannot exceed 20.
What is a nested data type and why isn't it the same as a document with depth>1?
nested data type allows to index arrays of objects and query their items individually via nested query. What this means is that Elasticsearch will index a document with such fields differently (see the page Nested Objects of the Definitive Guide for more explanation).
For instance, if in the following example we do not define "user" as nested field in the mapping, a query for user.first: John and user.last: White will return a match and it will be a mistake:
{
"group" : "fans",
"user" : [
{
"first" : "John",
"last" : "Smith"
},
{
"first" : "Alice",
"last" : "White"
}
]
}
If we do, Elasticsearch will index each item of the "user" list as an implicit sub-document and thus will use more resources, more disk and memory. This is why there is also another setting on the mappings: index.mapping.nested_fields.limit regulates how many different nested fields one can declare (which defaults to 50). To customize this you can see this answer.
So, Elasticsearch documents with depth > 1 are not indexed as nested unless you explicitly ask it to do so, and that's the difference.
Can I have nested fields inside nested?
Yes, you can! Just to stop this confusion, yes, you can define a nested field inside nested field in a mapping. It will look something like this:
PUT my_index
{
"mappings": {
"_doc": {
"properties": {
"user": {
"type": "nested",
"properties": {
"name": {
"type": "keyword"
},
"cars": {
"type": "nested",
"properties": {
"brand": {
"type": "keyword"
}
}
}
}
}
}
}
}
}
But keep in mind that the amount of implicit documents to be indexed will be multiplied, and it will be simply not that efficient.
Can I get the depth of my JSON objects from Kibana?
Most likely you can do it with scripts, check this blog post for further details: Using Painless in Kibana scripted fields.