Using hint to increase specific query's performance on indexed collections

hint_performance.js

// Suppose, we have 10000000 documents stored in bar collections (foo database)
use foo
for(var i = 0; i< 10000000; i++)(db.bar.insert({'a':i, 'b':i, 'c':i}))

// We create indexes for them 
db.bar.ensureIndex({'a':1})
db.bar.ensureIndex({'b':1})
db.bar.ensureIndex({'c':1})
db.bar.ensureIndex({'a':1, 'b':1,'c':1})

// Let query a number of document using $gt, $lt, sort the result & explain it. Note the number of scanned document & milis.
db.bar.find({'a':{$gt:500}, 'b':{$lt:500000}, 'c': 350555}).sort({'a':1}).explain()

// Let's use hint to speed up the above query. We'll hint 'a' 1st and see what happen next
db.bar.find({'a':{$gt:500}, 'b':{$lt:500000}, 'c': 350555}).sort({'a':1}).hint({'a':1}).explain()

// It was slightly better. What about we hint 'b' now
db.bar.find({'a':{$gt:500}, 'b':{$lt:500000}, 'c': 350555}).sort({'a':1}).hint({'b':1}).explain()

// Looks better. Let's hint 'c'
db.bar.find({'a':{$gt:500}, 'b':{$lt:500000}, 'c': 350555}).sort({'c':1}).hint({'b':1}).explain()

// Perfect ! Now, we should understand which index we should hint to increase our query's performance :D