Each document indexed is associated with an id and a type. The _id field can be used to index just the id, and possible also store it. By default it is not indexed and not stored (thus, not created).
Note, even though the _id is not indexed, all the APIs still work (since they work with the _uid field), as well as fetching by ids using the ids query/filter.
The _id field can be enabled to be indexed, and possibly stored, using:
{
"tweet" : {
:ref:`_id" : {"index <es-guide-reference-mapping>` ex <es-guide-reference-mapping>` "not_analyzed", "store" : "yes"}
}
}
In order to maintain backward compatibility, a node level setting index.mapping._id.indexed can be set to true to make sure that the id is indexed when upgrading to 0.16, though its recommended to not index the id.