Elasticsearch is a search engine that is built on top of Lucene and passes data to Lucene for storage and searching. Lucene data structures can perform in a better way if the data is stored in dense form; for example, all documents with the same type of fields can create a dense storage rather than storing different types of field in a single document. Lucene identifies documents with doc_id, which has an integer value and varies from 0 to total number of documents in the index. This is how Lucene recognizes Elasticsearch document in the index. These doc_id elements of Elasticsearch documents are used to communicate with Lucene's internal APIs.
For example, if we execute a match query on any term, Lucene will produce an iterator of doc_ids. These doc_ids elements are used to compute the score for the document in the search by retrieving the...
