Question

在MongoDB中是否可以对预定义数量的行进行聚合，而不是按分组。例如，我想计算每1000行的平均值，而不是按某一列分组。下表是一个较小的示例，我要计算每4个连续行的平均评分：

所以我的休息应该喜欢这样的东西：

以下是JSON中的输入数据：

[{"ItemName":"Item1","Rating":4},
{"ItemName":"Item2","Rating":4},
{"ItemName":"Item2","Rating":4},
{"ItemName":"Item3","Rating":2},
{"ItemName":"Item4","Rating":5},
{"ItemName":"Item5","Rating":4},
{"ItemName":"Item6","Rating":2},
{"ItemName":"Item7","Rating":4},
{"ItemName":"Item8","Rating":1},
{"ItemName":"Item9","Rating":4},
{"ItemName":"Item10","Rating":3},
{"ItemName":"Item11","Rating":2},
{"ItemName":"Item12","Rating":2}]

Answer 1

没有简单的方法。您将需要将整个集合分组为数组，这可能需要allowDiskUse才能处理具有巨大性能影响的大型数据集。

db.collection.aggregate([
    // count all documents
    { $group: {
        _id: null,
        cnt: { $sum: 1},
        docs: { $push: "$$ROOT" }
    } },
    // add _batch field to group documents by
    { $project: {
        _id: 0,
        docs: { $map: { 
            // add a sequential number to each
            input: { $zip: {
                inputs: [ "$docs",  { $range: [ 0, "$cnt" ] } ]
            } }, 
            as: "doc", 
            in: { $mergeObjects: [ 
                { $arrayElemAt: [ "$$doc", 0 ] }, 
                // split it in batches by 4 based on the sequential number
                { _batch: { $cond: [ 
                    { $eq: [ { $arrayElemAt: [  "$$doc", 1 ] }, 0 ] }, 
                    1, 
                    { $ceil: { $divide: [ { $arrayElemAt: [  "$$doc", 1 ] }, 4 ] } } 
                ] } }
            ] } 
        } }
    } },    
    { $unwind: "$docs" },
    { $replaceRoot: { newRoot: "$docs" } },
    // ensure original order, only if you need ItemRange as a string
    { $sort: { _id: 1 } },
    // calculate averages per batch
    { $group: { 
        _id: "$_batch",
        start: { $first: "$ItemName" }, // only if you need ItemRange as a string
        end: { $last: "$ItemName" }, // only if you need ItemRange as a string
        RatingAvg: {$avg: "$Rating"} 
    } },
    // only if you need them in order
    { $sort: { _id: 1 } },
    // calculate ItemRange, only if you need ItemRange as a string
    { $project: {
        _id: 0,
        ItemRange: { $concat: [ "$start", "-", "$end" ] },
        RatingAvg: 1 
    } },
])

不确定实际用例，因为当您删除例如第一个文件。

无论如何，如果您不需要格式为“ FirstName-LastName”的ItemRange并可以使用批号，则可以跳过2种内存中的持续排序，以提高性能。

MongoDB对预定义大小的行集合进行聚合

1 个答案: