db.test.insert({q: 'test', i: 1});
db.test.insert({q: 'shmest', i: 2});
db.test.insert({q: 'quest', i: 3});
db.test.ensureIndex({q: 1});
db.test.find({q: {$in: ['test', 'shmest']}});
//{ "_id" : ObjectId("5294c0b2c32bf844e1f74e52"), "q" : "shmest", "i" : 2 }
//{ "_id" : ObjectId("5294c09fc32bf844e1f74e51"), "q" : "test", "i" : 1 }

db.test.count({q: {$in: ['test', 'shmest']}});
//2

db.test.remove();

db.test.insert({query: 'test', i: 1});
db.test.insert({query: 'shmest', i: 2});
db.test.insert({query: 'quest', i: 3});
db.test.ensureIndex({query: 1});

db.test.find({query: {$in: ['test', 'shmest']}});
//query fails: no response (and on large collection it takes a lot of time as if there's no index)

db.test.count({query: {$in: ['test', 'shmest']}});
//2

db.test.find({query: 'test'});
//{ "_id" : ObjectId("5294c166c32bf844e1f74e55"), "query" : "test", "i" : 1 }

//db.test.find({query: 'shmest'});
{ "_id" : ObjectId("5294c166c32bf844e1f74e56"), "query" : "shmest", "i" : 2 }

db.test.find({$or: [{query: 'test'}, {query: 'shmest'}]});
//{ "_id" : ObjectId("5294c166c32bf844e1f74e55"), "query" : "test", "i" : 1 }
//{ "_id" : ObjectId("5294c166c32bf844e1f74e56"), "query" : "shmest", "i" : 2 }