可以避免使用$ elemMatch和常规查询在大型数组上进行$ unwind/聚合吗? [英] Possible to avoid $unwind / aggregation on large array using $elemMatch and regular query?

问题描述

我有一个类似于以下内容的文档集合(称为日志"):

I have a collection of documents (call it 'logs') which looks similar to this:

{
  "_id" : ObjectId("52f523892491e4d58e85d70a"),
  "ds_id" : "534d35d72491de267ca08e96",
  "eT" : NumberLong(1391784000),
  "vars" : [{
      "n" : "ActPow",
      "val" : 73.4186401367188,
      "u" : "kWh",
      "dt" : "REAL",
      "cM" : "AVE",
      "Q" : 99
    }, {
      "n" : "WinSpe",
      "val" : 3.06327962875366,
      "u" : "m/s",
      "dt" : "REAL",
      "cM" : "AVE",
      "Q" : 99
    }]
}

vars数组包含大约150个子文档，而不仅仅是上面显示的两个.我现在想做的是运行一个查询，该查询在上面显示的vars数组中检索两个子文档的val.

The vars array holds about 150 subdocuments, not just the two I have shown above. What I'd like to do now is to run a query which retrieves the val of the two subdocuments in the vars array that I have shown above.

使用聚合框架，我可以提出以下建议:

Using the aggregation framework, I've been able to come up with the following:

db.logs.aggregate( [ 
    { $match : 
        { ds_id: "534d35d72491de267ca08e96", 
          eT: { $lt : 1391784000 }, 
          vars: { $elemMatch: { n: "PowCrvVld", val: 3 }}
        }
    }, 
    { $unwind : "$vars" }, 
    { $match :
        { "vars.n" : { $in : ["WinSpe", "ActPow"] }}, 
        { $project : { "vars.n" : 1, N : 1}
    } 
]);

尽管这可行，但是在运行较大的查询时，我的内存已达到16MB的限制.鉴于我在vars数组中有大约150个子文档，因此我也想避免使用$unwind.

While this works, I run up against the 16MB limit when running larger queries. Seeing as I have about 150 subdocuments in the vars array, I'd also like to avoid $unwind if it's possible.

使用常规查询并使用$elemMatch，我已经能够检索其中一个值:

Using a regular query and using $elemMatch I have been able to retrieve ONE of the values:

db.logs.TenMinLog.find({
    ds_id : "534d35d72491de267ca08e96",
    eT : { $lt : 1391784000 },
    vars : { $elemMatch : { n : "PowCrvVld", val : 3 }
    }
}, {
    ds_id : 1,
    vars : { $elemMatch : { n : "ActPow", cM : "AVE" }
});

我的问题归结为，是否有一种方法可以在find的<projection>部分中对数组多次使用$ elemMatch.如果不是，是否有另一种方法可以轻松地检索这两个子文档而无需使用$ unwind?我也愿意接受其他一些我可能不会意识到的建议.谢谢！

What my question comes down to is if there's a way to use $elemMatch on an array multiple times in the <projection> part of find. If not, is there another way to easily retrieve those two subdocuments without using $unwind? I am also open to other suggestions that would be more performant that I may not be aware of. Thanks!

推荐答案

我强烈考虑迁移到MongoDB版本2.6.聚合已得到增强，可以返回一个游标，从而消除了16MB的文档限制:

I'd strongly consider a move to MongoDB version 2.6. Aggregation has been enhanced to return a cursor which eliminates the 16MB document limit:

在2.6版中进行了更改:

Changed in version 2.6:

db.collection.aggregate()方法返回一个游标，并且可以返回 任何大小的结果集.先前的版本会在 单个文档，并且结果集的大小限制为16 兆字节.

The db.collection.aggregate() method returns a cursor and can return result sets of any size. Previous versions returned all results in a single document, and the result set was subject to a size limit of 16 megabytes.

http://docs.mongodb.org/manual/core/aggregation-pipeline /

对于更复杂的聚合查询，您还可以发现许多增强功能:

Also there are a number of enhancements that you may find useful for more complex aggregation queries:

聚合增强

聚合管道增加了返回任何结果集的能力 大小，方法是返回游标或将输出写入 收藏.此外，聚合管道支持变量 并添加了新的操作来处理集合和修订数据.

The aggregation pipeline adds the ability to return result sets of any size, either by returning a cursor or writing the output to a collection. Additionally, the aggregation pipeline supports variables and adds new operations to handle sets and redact data.

db.collection.aggregate()现在返回一个游标，该游标启用 聚合管道返回任意大小的结果集.聚合 管道现在支持解释操作，以帮助分析 聚合操作.聚合现在可以使用更高效的 基于外部磁盘的排序过程.

The db.collection.aggregate() now returns a cursor, which enables the aggregation pipeline to return result sets of any size. Aggregation pipelines now support an explain operation to aid analysis of aggregation operations. Aggregation can now use a more efficient external-disk-based sorting process.

新的管道阶段:

• $ out阶段以输出到集合.
• $ redact阶段，以允许其他控制权来访问数据.

新的或修改的运算符:

• 设置表达式运算符.
• $ let和$ map运算符允许使用变量.
• $ literal运算符和$ size运算符.
• $ cond表达式现在接受一个对象或一个数组.

• set expression operators.
• $let and $map operators to allow for the use of variables.
• $literal operator and $size operator.
• $cond expression now accepts either an object or an array.

http://docs.mongodb.org/manual/release-notes/2.6 /

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