Flattened data type
editFlattened data type
editBy default, each subfield in an object is mapped and indexed separately. If the names or types of the subfields are not known in advance, then they are mapped dynamically.
The flattened
type provides an alternative approach, where the entire
object is mapped as a single field. Given an object, the flattened
mapping will parse out its leaf values and index them into one field as
keywords. The object’s contents can then be searched through simple queries
and aggregations.
This data type can be useful for indexing objects with a large or unknown number of unique keys. Only one field mapping is created for the whole JSON object, which can help prevent a mappings explosion from having too many distinct field mappings.
On the other hand, flattened object fields present a trade-off in terms of search functionality. Only basic queries are allowed, with no support for numeric range queries or highlighting. Further information on the limitations can be found in the Supported operations section.
The flattened
mapping type should not be used for indexing all
document content, as it treats all values as keywords and does not provide full
search functionality. The default approach, where each subfield has its own
entry in the mappings, works well in the majority of cases.
An flattened object field can be created as follows:
PUT bug_reports { "mappings": { "properties": { "title": { "type": "text" }, "labels": { "type": "flattened" } } } } POST bug_reports/_doc/1 { "title": "Results are not sorted correctly.", "labels": { "priority": "urgent", "release": ["v1.2.5", "v1.3.0"], "timestamp": { "created": 1541458026, "closed": 1541457010 } } }
During indexing, tokens are created for each leaf value in the JSON object. The values are indexed as string keywords, without analysis or special handling for numbers or dates.
Querying the top-level flattened
field searches all leaf values in the
object:
POST bug_reports/_search { "query": { "term": {"labels": "urgent"} } }
To query on a specific key in the flattened object, object dot notation is used:
POST bug_reports/_search { "query": { "term": {"labels.release": "v1.3.0"} } }
Supported operations
editBecause of the similarities in the way values are indexed, flattened
fields share much of the same mapping and search functionality as
keyword
fields.
Currently, flattened object fields can be used with the following query types:
-
term
,terms
, andterms_set
-
prefix
-
range
-
match
andmulti_match
-
query_string
andsimple_query_string
-
exists
When querying, it is not possible to refer to field keys using wildcards, as in
{ "term": {"labels.time*": 1541457010}}
. Note that all queries, including
range
, treat the values as string keywords. Highlighting is not supported on
flattened
fields.
It is possible to sort on an flattened object field, as well as perform simple
keyword-style aggregations such as terms
. As with queries, there is no
special support for numerics — all values in the JSON object are treated as
keywords. When sorting, this implies that values are compared
lexicographically.
Flattened object fields currently cannot be stored. It is not possible to
specify the store
parameter in the mapping.
Parameters for flattened object fields
editThe following mapping parameters are accepted:
Mapping field-level query time boosting. Accepts a floating point number,
defaults to |
|
|
The maximum allowed depth of the flattened object field, in terms of nested
inner objects. If a flattened object field exceeds this limit, then an
error will be thrown. Defaults to |
Should the field be stored on disk in a column-stride fashion, so that it
can later be used for sorting, aggregations, or scripting? Accepts |
|
Should global ordinals be loaded eagerly on refresh? Accepts |
|
Leaf values longer than this limit will not be indexed. By default, there is no limit and all values will be indexed. Note that this limit applies to the leaf values within the flattened object field, and not the length of the entire field. |
|
Determines if the field should be searchable. Accepts |
|
What information should be stored in the index for scoring purposes.
Defaults to |
|
A string value which is substituted for any explicit |
|
Which scoring algorithm or similarity should be used. Defaults
to |
|
|
Whether full text queries should split the input on
whitespace when building a query for this field. Accepts |