MongoDB Enterprise > db.t.explain().aggregate([{$group: {_id: "$item", min: {$min: "$price.a"}, max: {$max: "$price.a"}, f: {$first: "$price.a"}}}]).queryPlanner.winningPlan.slotBasedPlan.stages
[3] mkbson s27 [_id = s12, min = s25, max = s26, f = s24] true false
[3] project [s25 = fillEmpty (s16, null), s26 = fillEmpty (s20, null)]
[3] group [s12] [s16 = min (let [l1.0 = s15] if (! exists (l1.0) || typeMatch (l1.0, 0x00000440), Nothing, l1.0)), s20 = max (let [l2.0 = s19] if (! exists (l2.0) || typeMatch (l2.0, 0x00000440), Nothing, l2.0)), s24 = first (fillEmpty (s23, null))]
[3] traverse s23 s22 s21 [s10, s11, s12, s13, s15, s17, s19] {} {}
from
    [3] project [s21 = getField (s10, "price")]
    [3] traverse s19 s18 s17 [s10, s11, s12, s13, s15] {} {}
    from
        [3] project [s17 = getField (s10, "price")]
        [3] traverse s15 s14 s13 [s10, s11, s12] {} {}
        from
            [3] project [s13 = getField (s10, "price")]
            [3] project [s12 = fillEmpty (s11, null)]
            [3] project [s11 = getField (s10, "item")]
            [2] traverse s10 s9 s4 [s5] {} {}
            from
                [1] scan s4 s5 none none none none [] @"7fb2bafd-3565-4723-8153-e775e578c8c9" true false
            in
                [2] cfilter {isObject (s4)}
                [2] mkbson s9 s4 [item] keep [price = s8] true false
                [2] traverse s8 s7 s6 {} {}
                from
                    [2] project [s6 = getField (s4, "price")]
                    [2] limit 1
                    [2] coscan
                in
                    [2] cfilter {isObject (s6)}
                    [2] mkbson s7 s6 [a] keep [] true false
                    [2] limit 1
                    [2] coscan
        in
            [3] project [s14 = getField (s13, "a")]
            [3] limit 1
            [3] coscan
    in
        [3] project [s18 = getField (s17, "a")]
        [3] limit 1
        [3] coscan
in
    [3] project [s22 = getField (s21, "a")]
    [3] limit 1
    [3] coscan