_id

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 term, terms or prefix queries/filters (including the specific ids query/filter).

The _id field can be enabled to be indexed, and possibly stored, using the appropriate mapping attributes:

{
    "tweet" : {
        "_id" : {
            "index" : "not_analyzed",
            "store" : true
        }
    }
}

The _id mapping can also be associated with a path that will be used to extract the id from a different location in the source document. For example, having the following mapping:

{
    "tweet" : {
        "_id" : {
            "path" : "post_id"
        }
    }
}

Will cause 1 to be used as the id for:

{
    "message" : "You know, for Search",
    "post_id" : "1"
}

This does require an additional lightweight parsing step while indexing, in order to extract the id to decide which shard the index operation will be executed on.