Does Mongo select queries optimize?

Question

MongoDB

Let's say there are two SQL queries

select * from users
and
select username, email from users

As I understand it, they correspond to

> db.users.find();
and
> db.users.find( { }, { username: 1, email: 1 } );

Question: Does Mongo optimize these queries? Does he have a time difference for these requests?

Answer 1

Full sample

{
  "cursor" : "BasicCursor",
  "nscanned" : 1136854,
  "nscannedObjects" : 1136854,
  "n" : 1136854,
  "millis" : 45205,
  "nYields" : 0,
  "nChunkSkips" : 0,
  "isMultiKey" : false,
  "indexOnly" : false,
  "indexBounds" : {
    
  }
}

Selecting one field

{
  "cursor" : "BasicCursor",
  "nscanned" : 1136854,
  "nscannedObjects" : 1136854,
  "n" : 1136854,
  "millis" : 40294,
  "nYields" : 0,
  "nChunkSkips" : 0,
  "isMultiKey" : false,
  "indexOnly" : false,
  "indexBounds" : {
    
  }
}

The real profit will be if only the fields in the index are selected.

Answer 2

you can tritely execute a bunch of such requests and detect the time =) you will visually see the difference, at the same time tell us here.

Answer 3

If you only need 2 fields, then the second option is better, since in the first one you will send the entire collection to the client.
I don’t think that mongo optimizes anything, since there is nothing to optimize here - in both cases, you need to bypass the entire collection, just in the second case, the amount of data sent is less.