Muilti key Indexes on mongodb

multi_key_index.js

// Let's say we have a database named as 'foo'
use foo

// On the foo database, there is a collection named as 'bar' & we'll add a record on 'bar' collection
db.bar.insert({'a':1, 'b':5})

// Now, we are going to add indexes for both a & b
db.bar.ensureIndex({'a':1, 'b': -1})

// Let's see which index the bar uses when querying it a or/and b. Notice what is the values of "cursor" & "isMultiKey" in the returned result 
db.bar.find({'a':50, 'b':20}).explain()

// Next, let's insert a document where a's value is an array
db.bar.insert({'a':[1,2,3], 'b':4})

// Then, let's query them and also explain to us what does it do ( notice the "cursor" & "isMultiKey" properties in the result)
db.bar.find({'a':1}).explain()

// Finally, let's add a new document, where both a & b contain array values. See how it goes later 
db.bar.insert({'a':[4,5], 'b':[7,8,9]})

// Supposed, we have a new document that has property 'c' where 'c' contain array of 'd's
db.bar.insert({'a':2, 'b':7, 'c': [{'d':1}, {'d':2},{'d':3},{'d':4}]})

// Next, let's add indexes to a,b & c.d 
db.bar.ensureIndex({'a':1, 'b':1, 'c.d':1})

// Let's query them again 
db.bar.find({'a':{$in:[1,2]}, 'b':{$gt:3}, 'c.d':1})

// Explain what does the prior query did ( notice the "cursor" & "isMultiKey" properties in the result)
db.bar.find({'a':{$in:[1,2]}, 'b':{$gt:3}, 'c.d':1}).explain()