A pipeline begins [{$match}, {$sort}] where $match filters on field a and $sort is on field b, with a compound index {a:1, b:1}. The explain shows no in-memory SORT stage at all. What internal property of the index makes the sort free here?
// index: { a: 1, b: 1 }
db.c.aggregate([
{ $match: { a: 42 } },
{ $sort: { b: 1 } }
])