MongoDB按需物化视图介绍

2021-12-31 本文已影响0人 MongoDB中文社区

注意

本页的内容讨论了按需物化视图。有关视图的讨论，请参阅视图。

从4.2版本开始，MongoDB为aggregation pipeline添加了$merge阶段。此阶段可以将管道结果合并到现有集合中，而不是完全替换现有集合。此功能允许用户创建按需物化视图，每次运行管道时都可以更新输出集合的内容。

示例

假设现在接近2019年1月末，集合bakesales包含按项目分类的销售信息：

db.bakesales.insertMany( [ { date: new ISODate("2018-12-01"), item: "Cake - Chocolate", quantity: 2, amount: new NumberDecimal("60") }, { date: new ISODate("2018-12-02"), item: "Cake - Peanut Butter", quantity: 5, amount: new NumberDecimal("90") }, { date: new ISODate("2018-12-02"), item: "Cake - Red Velvet", quantity: 10, amount: new NumberDecimal("200") }, { date: new ISODate("2018-12-04"), item: "Cookies - Chocolate Chip", quantity: 20, amount: new NumberDecimal("80") }, { date: new ISODate("2018-12-04"), item: "Cake - Peanut Butter", quantity: 1, amount: new NumberDecimal("16") }, { date: new ISODate("2018-12-05"), item: "Pie - Key Lime", quantity: 3, amount: new NumberDecimal("60") }, { date: new ISODate("2019-01-25"), item: "Cake - Chocolate", quantity: 2, amount: new NumberDecimal("60") }, { date: new ISODate("2019-01-25"), item: "Cake - Peanut Butter", quantity: 1, amount: new NumberDecimal("16") }, { date: new ISODate("2019-01-26"), item: "Cake - Red Velvet", quantity: 5, amount: new NumberDecimal("100") }, { date: new ISODate("2019-01-26"), item: "Cookies - Chocolate Chip", quantity: 12, amount: new NumberDecimal("48") }, { date: new ISODate("2019-01-26"), item: "Cake - Carrot", quantity: 2, amount: new NumberDecimal("36") }, { date: new ISODate("2019-01-26"), item: "Cake - Red Velvet", quantity: 5, amount: new NumberDecimal("100") }, { date: new ISODate("2019-01-27"), item: "Pie - Chocolate Cream", quantity: 1, amount: new NumberDecimal("20") }, { date: new ISODate("2019-01-27"), item: "Cake - Peanut Butter", quantity: 5, amount: new NumberDecimal("80") }, { date: new ISODate("2019-01-27"), item: "Tarts - Apple", quantity: 3, amount: new NumberDecimal("12") }, { date: new ISODate("2019-01-27"), item: "Cookies - Chocolate Chip", quantity: 12, amount: new NumberDecimal("48") }, { date: new ISODate("2019-01-27"), item: "Cake - Carrot", quantity: 5, amount: new NumberDecimal("36") }, { date: new ISODate("2019-01-27"), item: "Cake - Red Velvet", quantity: 5, amount: new NumberDecimal("100") }, { date: new ISODate("2019-01-28"), item: "Cookies - Chocolate Chip", quantity: 20, amount: new NumberDecimal("80") }, { date: new ISODate("2019-01-28"), item: "Pie - Key Lime", quantity: 3, amount: new NumberDecimal("60") }, { date: new ISODate("2019-01-28"), item: "Cake - Red Velvet", quantity: 5, amount: new NumberDecimal("100") },] );

定义按需物化视图

下面的updateMonthlySales函数定义了一个monthlybakesales物化视图，其中包含累积的每月销售信息。在示例中，该函数采用了一个日期参数来更新从特定日期开始的每月销售信息。

updateMonthlySales = function(startDate) { db.bakesales.aggregate( [ { $match: { date: { $gte: startDate } } }, { $group: { _id: { $dateToString: { format: "%Y-%m", date: "$date" } }, sales_quantity: { $sum: "$quantity"}, sales_amount: { $sum: "$amount" } } }, { $merge: { into: "monthlybakesales", whenMatched: "replace" } } ] );};

$match阶段过滤数据以仅处理那些销售额大于或等于startDate

阶段按年-月对销售信息进行分组。此阶段输出的文档具有以下形式：

{ "_id" : "<YYYY-mm>", "sales_quantity" : <num>, "sales_amount" : <NumberDecimal> }

$merge阶段将输出写入到monthlybakesales集合

基于on_id字段（未分片输出集合的默认值），此阶段会检查聚合结果中的文档是否匹配集合中的现有文档：

当匹配时（即同年月的文档已经存在于集合中），此阶段会使用来自聚合结果的文档替换现有文档；

当不匹配时，此阶段将聚合结果中的文档插入到集合中（不匹配时的默认行为）。

执行初始运行

对于初始运行，你可以传入一个日期new ISODate("1970-01-01")：

updateMonthlySales(new ISODate("1970-01-01"));

初始运行后，monthlybakesales包含以下文档；即db.monthlybakesales.find().sort( { _id: 1 } )返回以下内容：

{ "_id" : "2018-12", "sales_quantity" : 41, "sales_amount" : NumberDecimal("506") }{ "_id" : "2019-01", "sales_quantity" : 86, "sales_amount" : NumberDecimal("896") }

刷新物化视图

假设到了2019年2月的第一周，bakesales集合更新了新的销售信息；具体来说就是一月和二月新增的销售。

db.bakesales.insertMany( [ { date: new ISODate("2019-01-28"), item: "Cake - Chocolate", quantity: 3, amount: new NumberDecimal("90") }, { date: new ISODate("2019-01-28"), item: "Cake - Peanut Butter", quantity: 2, amount: new NumberDecimal("32") }, { date: new ISODate("2019-01-30"), item: "Cake - Red Velvet", quantity: 1, amount: new NumberDecimal("20") }, { date: new ISODate("2019-01-30"), item: "Cookies - Chocolate Chip", quantity: 6, amount: new NumberDecimal("24") }, { date: new ISODate("2019-01-31"), item: "Pie - Key Lime", quantity: 2, amount: new NumberDecimal("40") }, { date: new ISODate("2019-01-31"), item: "Pie - Banana Cream", quantity: 2, amount: new NumberDecimal("40") }, { date: new ISODate("2019-02-01"), item: "Cake - Red Velvet", quantity: 5, amount: new NumberDecimal("100") }, { date: new ISODate("2019-02-01"), item: "Tarts - Apple", quantity: 2, amount: new NumberDecimal("8") }, { date: new ISODate("2019-02-02"), item: "Cake - Chocolate", quantity: 2, amount: new NumberDecimal("60") }, { date: new ISODate("2019-02-02"), item: "Cake - Peanut Butter", quantity: 1, amount: new NumberDecimal("16") }, { date: new ISODate("2019-02-03"), item: "Cake - Red Velvet", quantity: 5, amount: new NumberDecimal("100") }] )

为了刷新1月和2月的monthlybakesales数据，需要再次运行该函数以重新运行聚合管道，日期参数值从new ISODate("2019-01-01")开始。

updateMonthlySales(new ISODate("2019-01-01"));

monthlybakesales的内容已更新，并能反映出bakesales集合中的最新数据；即db.monthlybakesales.find().sort( { _id: 1 } )返回以下内容：

{ "_id" : "2018-12", "sales_quantity" : 41, "sales_amount" : NumberDecimal("506") }{ "_id" : "2019-01", "sales_quantity" : 102, "sales_amount" : NumberDecimal("1142") }{ "_id" : "2019-02", "sales_quantity" : 15, "sales_amount" : NumberDecimal("284") }