MongoDB 中Aggregate使用与相关限制

MongoDB分组怎么用?

官网：https://docs.mongodb.com/manual/reference/operator/aggregation-pipeline/

不指定字段分组

案例1：

db.getCollection('6117Decartes').aggregate( 
   [
    { 
    $group : { 
        _id : null,
        "count":{$sum:1} } 
    } 
   ],
   {
        allowDiskUse: true
    } 
);

要注意的是，字段可被统计才行，否则会报错​​，看个例子。​​‍

代码 InstitutionID 是非统计字段

db.getCollection('6117Decartes').aggregate( 
   [
    { 
    $group : { 
        _id : null,
        "InstitutionID":"$InstitutionID"
        } 
    } 
   ],
   {
        allowDiskUse: true
    } 
);
正确的写法是：
db.getCollection('6117Decartes').aggregate(
   [
    {
    $group : {
        _id : null,
        "InstitutionID":{$max:"$InstitutionID"}
        }
    }
   ],
   {
        allowDiskUse: true
    }
);

否则报错如下：

案例2：

db.sales.insert([
    { "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-03-01T08:00:00Z") },
    { "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-03-01T09:00:00Z") },
    { "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-03-15T09:00:00Z") },
    { "_id" : 4, "item" : "xyz", "price" : 5, "quantity" : 20, "date" : ISODate("2014-04-04T11:21:39.736Z") },
    { "_id" : 5, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-04-04T21:23:13.331Z") }
]);

不指定数据分组

db.sales.aggregate(
   [
      {
        $group : {
           _id : null,
           minPrice:{$min:"$price"},    //最小值
           maxPrice:{$max:"$price"},    //最大值
           avgPrice:{ $avg: "$price" }, //平均值
           totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } }, //总价 $sum(price * quantity)
           lastItem:{$last:"$item"},    //取最后一条
           count: { $sum: 1 } // 总条目数
        }
      }
   ]
);

多字段分组

db.getCollection('6117Decartes').aggregate( 
   [
    { 
    $group : { 
        _id : {"InstitutionID":"$InstitutionID","logtype":"$logtype"},
        "总条目数":{$sum:1} } 
    } 
   ],
   {
        allowDiskUse: true
    } 
);

案例2：

db.sales.aggregate( 
   [
    { 
    $group : { 
        _id : {"商品":"$item", "价格":"$price"},
        "总条目数":{$sum:1} } 
    } 
   ] 
);

将 $push 聚合分组指定到列结构到数组中

db.books.insert([
    { "_id" : 8751, "title" : "The Banquet", "author" : "Dante", "copies" : 2 },
    { "_id" : 8752, "title" : "Divine Comedy", "author" : "Dante", "copies" : 1 },
    { "_id" : 8645, "title" : "Eclogues", "author" : "Dante", "copies" : 2 },
    { "_id" : 7000, "title" : "The Odyssey", "author" : "Homer", "copies" : 10 },
    { "_id" : 7020, "title" : "Iliad", "author" : "Homer", "copies" : 10 }
]);
db.books.aggregate(
   [
     { $group : { _id : "$author", books: { $push: "$title" } } }
   ]
);

可能出现的问题：分组内存使用超过限制时错误

{
"message" : "Exceeded memory limit for $group, but didn't allow external sort. Pass allowDiskUse:true to opt in.",
"stack" : "MongoError: Exceeded memory limit for $group, but didn't allow external sort. Pass allowDiskUse:true to opt in." +
              "at queryCallback (/tmp/.mount_nosqlbO7RhZG/app/resources/app.asar/node_modules/mongodb-core/lib/cursor.js:223:25)" +
              "at /tmp/.mount_nosqlbO7RhZG/app/resources/app.asar/node_modules/mongodb-core/lib/connection/pool.js:541:18" +
              "at _combinedTickCallback (internal/process/next_tick.js:131:7)" +
              "at process._tickCallback (internal/process/next_tick.js:180:9)",
"name" : "MongoError",
"ok" : 0,
"errmsg" : "Exceeded memory limit for $group, but didn't allow external sort. Pass allowDiskUse:true to opt in.",
"code" : 16945,
"codeName" : "Location16945"
}

怎么解决？

• id 字段是必须要的，如果不指定字段进行分组则用 null,表示不分组的统计；
• 分组内存使用限制是100M,默认情况下如果超过了限制100M则会出现错误。如果想对超过100M的大数据进行处理，可以使用 allowDiskUse 选项来进行分组时写到磁盘临时文件中处理。

db.getCollection('6117Decartes').aggregate( 
   [
    { 
    $group : { 
        _id : {"InstitutionID":"$InstitutionID","logtype":"$logtype"},
        "总条目数":{$sum:1} } 
    } 
   ],
   {
        allowDiskUse: true
    } 
);