Efficient way to retrieve all _ids in ElasticSearch

Question

What is the fastest way to get all _ids of a certain index from ElasticSearch? Is it possible by using a simple query? One of my index has around 20,000 documents.

Answer 1

Another option

curl 'http://localhost:9200/index/type/_search?pretty=true&fields='

will return _index, _type, _id and _score.