mongodb index and exists

mongodb_index_and_exists.js

// Suppose we have a collection "my_coll" with documents that look like this (we're talking about things that have positions on chromosomes):
// { _id: "1_123",
//   chr: "1",
//   pos: 123,
//   accession_in_external_db: "an_id"
//  },
// { _id: "1_456",
//   chr: "1",
//   pos: 456
//  },
// { _id: "1_789",
//   chr: "1",
//   pos: 789,
//   accession_in_external_db: "another_id"
//  }
// 
// Notice that the second document does _not_ have an accession_in_external_db.
//
// Get count of documents with an accession_in_external_db:
db.my_coll.find({'accession_in_external_db': {$exists: true}}).count()
// => 11604013

// But now create an index and try it again:
db.my_coll.ensureIndex({accession_in_external_db: 1})
db.my_coll.find({'accession_in_external_db': {$exists: true}}).count()
// => 0