在中间管道中，在$ addFields和$ project之间有更好的性能

Question

我想知道mongodb聚合中的$addFields或$project中间阶段之间是否存在性能差异

让我们假设这些数据：

{"_id":"5c9b88648a2b5700118e5a05","values":[5,8,2],"piv":1}
{"_id":"5c9b88658a2b5700118e5a06","values":[9,8,2],"piv":1}
{"_id":"5c9b88658a2b5700118e5a07","values":[1,8,2],"piv":2}
{"_id":"5c9b88658a2b5700118e5a08","values":[5,3,2],"piv":2}

这两个聚合之间是否存在性能差异（这只是一个示例）：

db.test.aggregate([{
    // This is the stage that will change
    $addFields: {
        v: {
            $reduce: {
                input: '$values',
                initialValue: 0,
                in: {
                    $sum: ["$$value", "$$this"]
                }
            }
        }
    }
}, {
    $group: {
        _id: '$piv',
        vals: {
            $sum: '$v'
        }
    }
}])

和

db.test.aggregate([{
    // This is the stage that will change
    $project: {
        piv: 1,
        v: {
            $reduce: {
                input: '$values',
                initialValue: 0,
                in: {
                    $sum: ["$$value", "$$this"]
                }
            }
        }
    }
}, {
    $group: {
        _id: '$piv',
        vals: {
            $sum: '$v'
        }
    }
}])