I have a compound index:

{ 
    field1: 1, 
    field2: 1, 
    field3: 1 
}

When I do a query:

{ 
    field1: { $in: ['A', 'B'] },
    field2: { $in: ['C', 'D'] },
    field3: null
}

which hit the index field1_1_field2_1_field3_1, and in executionStats' IXSCAN step, field3's range is [null, null].

BUT, { field3:

} exists in FETCH step ??? I have indexed it and use it in IXSCAN step, why mongo need to filter it again ?