Elasticsearch 101: Object vs Nested

📅 Published: Apr 13, 2025 • Thomas Queste

Interview question on Elasticsearch: what is the difference between object and nested data types?

TL;DR

object: Flattens fields, losing associations between fields in the same object
nested: Preserves object integrity by creating hidden documents for each object
nested is required to relate fields together
nested has performance costs
flattened is an alternative for arbitrary keys (all values treated as keywords)

The Differences

Object:

Flattens the structure (field names are concatenated)
Relationships between fields in the same object are lost

Nested:

Store each nested object as a separate hidden document
Preserves relationships between fields
Requires special query syntax (nested query)

Example

PUT my_index
{
  "mappings": {
    "properties": {
      "attributes": {
        "type": "nested",
        "properties": {
          "code": { "type": "text" },
          "value": { "type": "keyword" }
        }
      },
    }
  }
}

This enables to query for specific combinations of code and value within the same object.

For example, we want all documents with an attribute of code: color and value: red.
This is not possible with object type, as it flattens the structure.

GET my_index/_search
{
  "query": {
    "nested": {
      "path": "attributes",
      "query": {
        "bool": {
          "must": [
            { "match": { "attributes.code": "color" }},
            { "match": { "attributes.value": "red" }}
          ]
        }
      }
    }
  }
}

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