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:
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.
- $cond expression now accepts either an object or an array.
http://docs.mongodb.org/manual/release-notes/2.6 /
这篇关于可以避免使用$ elemMatch和常规查询在大型数组上进行$ unwind/聚合吗?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！