Why does Mongo hint make a query run up to 10 times faster?

Question

If I run a mongo query from the shell with explain(), get the name of the index used and then run the same query again, but with hint() specifying the same index to be used - "millis" field from explain plan is decreased significantly

for example

no hint provided:

>>db.event.find({ "type" : "X", "active" : true, "timestamp" : { "$gte" : NumberLong("1317498259000") }, "count" : { "$gte" : 0 } }).limit(3).sort({"timestamp" : -1 }).explain();

{
    "cursor" : "BtreeCursor my_super_index",
    "nscanned" : 599,
    "nscannedObjects" : 587,
    "n" : 3,
    "millis" : 24,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "isMultiKey" : true,
    "indexOnly" : false,
    "indexBounds" : { ... }
} 

hint provided:

>>db.event.find({ "type" : "X", "active" : true, "timestamp" : { "$gte" : NumberLong("1317498259000") }, "count" : { "$gte" : 0 } }).limit(3).sort({"timestamp" : -1 }).hint("my_super_index").explain();

{
    "cursor" : "BtreeCursor my_super_index",
    "nscanned" : 599,
    "nscannedObjects" : 587,
    "n" : 3,
    "millis" : 2,
    "nYields" : 0,
    "nChunkSkips" : 0,
    "isMultiKey" : true,
    "indexOnly" : false,
    "indexBounds" : { ... }
} 

The only difference is "millis" field

Does anyone know why is that?

UPDATE: "Selecting which index to use" doesn't explain it, because mongo, as far as I know, selects index for each X (100?) runs, so it should be as fast as with hint next (X-1) runs

Answer 1

Mongo uses an algorithm to determine which index to be used when no hint is provided and then caches the index used for the similar query for next 1000 calls

But whenever you explain a mongo query it will always run the index selection algorithm, thus the explain() with hint will always take less time when compared with explain() without hint.

Similar question was answered here Understanding mongo db explain