Original summary

Sort function does not use the compound index

Original Description

This is the collection schema:

{
    a: 11,
    b: 22,
    c: 33
}

The index is:

{
    a: 1,
    b: 1,
    c: 1
}

If the query is:

db.test_coll.find({a: 11, b: 22}).sort({ c: 1}).explain()

I can see that there is no SORT stage which is find.
But, If I remove the b value from the query:

db.test_coll.find({a: 11}).sort({ c: 1}).explain()

The queryPlanner adds another SORT stage which is very bad for performance issues.

In general, I noticed that when mongo uses minKey and maxKey index bounds, it doesn't use the index for sorting.

If I'll use $in for b value with all the possible values, it will use the index correctly also for sorting.
But if it uses the minKey and maxKey values, it does't use the index for sorting.
I don't understand what is the difference between the two.
Looks like a bug.