CouchDB - Map Reduce类似于SQL Group by

Question

考虑下面存储在CouchDB中的示例文档

 {
"_id":....,
"rev":....,
"type":"orders",
"Period":"2013-01",
"Region":"East",
"Category":"Stationary",
"Product":"Pen",
"Rate":1,
"Qty":10,
"Amount":10
}

{
"_id":....,
"rev":....,
"type":"orders",
"Period":"2013-02",
"Region":"South",
"Category":"Food",
"Product":"Biscuit",
"Rate":7,
"Qty":5,
"Amount":35
}

考虑以下SQL查询

SELECT Period, Region,Category, Product, Min(Rate),Max(Rate),Count(Rate), Sum(Qty),Sum(Amount)
FROM Sales
GROUP BY Period,Region,Category, Product;

是否可以在couchdb中创建与上述SQL查询等效的map/reduce视图,并生成类似的输出

[
    {
        "Period":"2013-01",
        "Region":"East",
        "Category":"Stationary",
        "Product":"Pen",
        "MinRate":1,
        "MaxRate":2,
        "OrdersCount":20,
        "TotQty":1000,
        "Amount":1750
    },
    {
    ... 
    }

]

Answer 1

首先，我相信@benedolph 的答案是最佳实践和最佳案例场景。理想情况下，每个归约应返回 1 个标量值，以使代码尽可能简单。

但是，您确实必须发出多个查询才能检索问题描述的完整结果集。如果您无法选择并行运行查询，或者减少查询数量非常重要，则可以一次完成所有操作。

您的地图功能将非常简单：

function (doc) {
    emit([ doc.Period, doc.Region, doc.Category, doc.Product ], doc);
}

减少函数是它变得冗长的地方：

function (key, values, rereduce) {
    // helper function to sum all the values of a specified field in an array of objects
    function sumField(arr, field) {
        return arr.reduce(function (prev, cur) {
            return prev + cur[field];
        }, 0);
    }

    // helper function to create an array of just a single property from an array of objects
    // (this function came from underscore.js, at least it's name and concept)
    function pluck(arr, field) {
        return arr.map(function (item) {
            return item[field];
        });
    }

    // rereduce made this more challenging, and I could not thoroughly test this right now
    // see the CouchDB wiki for more information
    if (rereduce) {
        // a rereduce handles transitionary values
        // (so the "values" below are the results of previous reduce functions, not the map function)
        return {
            OrdersCount: sumField(values, "OrdersCount"),
            MinRate: Math.min.apply(Math, pluck(values, "MinRate")),
            MaxRate: Math.max.apply(Math, pluck(values, "MaxRate")),
            TotQty: sumField(values, "TotQty"),
            Amount: sumField(values, "Amount")
        };
    } else {
        var rates = pluck(values, "Rate");

        // This takes a group of documents and gives you the stats you were asking for
        return {
            OrdersCount: values.length,
            MinRate: Math.min.apply(Math, rates),
            MaxRate: Math.max.apply(Math, rates),
            TotQty: sumField(values, "Qty"),
            Amount: sumField(values, "Amount")
        };
    }
}

我根本无法测试这段代码的“rereduce”分支，你必须自己测试。（但这应该有效）有关减少与重新减少的信息，请参阅wiki 。

我在顶部添加的辅助函数实际上使代码整体更短且更易于阅读，它们很大程度上受到我使用Underscore.js 的经验的影响。但是，您不能在reduce 函数中包含CommonJS 模块，因此必须手动编写。

同样，最好的情况是让每个聚合字段都有自己的映射/归约索引，但如果您无法选择，则上面的代码应该可以为您提供问题中所描述的内容。