Efficient way to retrieve all _ids in ElasticSearch

For elasticsearch 5.x, you can use the "_source" field.

GET /_search
{
    "_source": false,
    "query" : {
        "term" : { "user" : "kimchy" }
    }
}

"fields" has been deprecated. (Error: "The field [fields] is no longer supported, please use [stored_fields] to retrieve stored fields or _source filtering if the field is not stored")

Elaborating on answers by Robert Lujo and Aleck Landgraf, if you want the IDs in a list from the returned generator, here is what I use:

from elasticsearch import Elasticsearch
from elasticsearch import helpers


es = Elasticsearch(hosts=[YOUR_ES_HOST])
hits = helpers.scan(
    es,
    query={"query":{"match_all": {}}},
    scroll='1m',
    index=INDEX_NAME
)
    
ids = [hit['_id'] for hit in hits]

Edit: Please also read the answer from Aleck Landgraf

You just want the elasticsearch-internal _id field? Or an id field from within your documents?

For the former, try

curl http://localhost:9200/index/type/_search?pretty=true -d '
{ 
    "query" : { 
        "match_all" : {} 
    },
    "stored_fields": []
}
'

Note 2017 Update: The post originally included "fields": [] but since then the name has changed and stored_fields is the new value.

The result will contain only the "metadata" of your documents

{
  "took" : 7,
  "timed_out" : false,
  "_shards" : {
    "total" : 5,
    "successful" : 5,
    "failed" : 0
  },
  "hits" : {
    "total" : 4,
    "max_score" : 1.0,
    "hits" : [ {
      "_index" : "index",
      "_type" : "type",
      "_id" : "36",
      "_score" : 1.0
    }, {
      "_index" : "index",
      "_type" : "type",
      "_id" : "38",
      "_score" : 1.0
    }, {
      "_index" : "index",
      "_type" : "type",
      "_id" : "39",
      "_score" : 1.0
    }, {
      "_index" : "index",
      "_type" : "type",
      "_id" : "34",
      "_score" : 1.0
    } ]
  }
}

For the latter, if you want to include a field from your document, simply add it to the fields array

curl http://localhost:9200/index/type/_search?pretty=true -d '
{ 
    "query" : { 
        "match_all" : {} 
    },
    "fields": ["document_field_to_be_returned"]
}
'

Better to use scroll and scan to get the result list so elasticsearch doesn't have to rank and sort the results.

With the elasticsearch-dsl python lib this can be accomplished by:

from elasticsearch import Elasticsearch
from elasticsearch_dsl import Search

es = Elasticsearch()
s = Search(using=es, index=ES_INDEX, doc_type=DOC_TYPE)

s = s.fields([])  # only get ids, otherwise `fields` takes a list of field names
ids = [h.meta.id for h in s.scan()]

Console log:

GET http://localhost:9200/my_index/my_doc/_search?search_type=scan&scroll=5m [status:200 request:0.003s]
GET http://localhost:9200/_search/scroll?scroll=5m [status:200 request:0.005s]
GET http://localhost:9200/_search/scroll?scroll=5m [status:200 request:0.005s]
GET http://localhost:9200/_search/scroll?scroll=5m [status:200 request:0.003s]
GET http://localhost:9200/_search/scroll?scroll=5m [status:200 request:0.005s]
...

Note: scroll pulls batches of results from a query and keeps the cursor open for a given amount of time (1 minute, 2 minutes, which you can update); scan disables sorting. The scan helper function returns a python generator which can be safely iterated through.