[[mapping-tier-field]]
=== `_tier` field

When performing queries across multiple indexes, it is sometimes desirable to
target indexes held on nodes of a given data tier (`data_hot`, `data_warm`, `data_cold` or `data_frozen`).
The `_tier` field allows matching on the `tier_preference` setting of the index a document was indexed into.
The preferred value is accessible in certain queries :

[source,console]
--------------------------
PUT index_1/_doc/1
{
  "text": "Document in index 1"
}

PUT index_2/_doc/2?refresh=true
{
  "text": "Document in index 2"
}

GET index_1,index_2/_search
{
  "query": {
    "terms": {
      "_tier": ["data_hot", "data_warm"] <1>
    }
  }
}
--------------------------

<1> Querying on the `_tier` field


Typically a query will use a `terms` query to list the tiers of interest but you can use
the `_tier` field in any query that is rewritten to a `term` query, such as the
`match`,  `query_string`, `term`, `terms`, or `simple_query_string` query, as well as `prefix`
and `wildcard` queries. However, it does not support `regexp` and `fuzzy`
queries.

The `tier_preference` setting of the index is a comma-delimited list of tier names
in order of preference i.e. the preferred tier for hosting an index is listed first followed
by potentially many fall-back options. Query matching only considers the first preference
(the first value of a list).