为什么 MongoDb 排序在查找集合中很慢 [英] Why MongoDb sort is slow with lookup collections

问题描述

我的 mongodb 数据库中有两个集合，如下所示:

I have two collections in my mongodb database as follows:

employee_details 包含大约 330000 个文档，其中 department_id 作为部门集合的参考

部门 集合，包含 2 个字段 _id 和 dept_name

我想通过查找方法使用department_id作为外键加入上述两个集合.加入工作正常，但当我添加排序时，mongo 查询执行需要很长时间.

I want to join the above two collections using department_id as foreign key by using lookup method. Join works fine but the mongo query execution takes long time when I add sort.

注意:如果我删除排序对象或删除查找方法，执行速度会很快.

Note: The execution is fast If I remove the sort object or If I remove the lookup method.

我在不同的博客和 SO 中引用了几篇文章，但没有一个给出排序的解决方案.

I have referred several posts in different blogs and SO, but none of them give a solution with sort.

我的查询如下:

db.getCollection("employee_details").aggregate([
  {
    $lookup: {
      from: "departments",
      localField: "department_id",
      foreignField: "_id",
      as: "Department"
    }
  },
  { $unwind: { path: "$Department", preserveNullAndEmptyArrays: true } },
  { $sort: { employee_fname: -1 } },
  { $limit: 10 }
]);
 

有人可以提供一种方法来使上述查询毫不延迟地工作，因为我的客户不能在性能延迟上妥协.我希望有一些方法可以解决性能问题，因为 nosql 旨在处理大型数据库.

Can someone give a method to make the above query to work without delay, as my client cannot compromise with the performance delay. I hope there is some method to fix the performance issue as nosql is intented to handle large database.

那里有可用的索引方法吗?这样我就可以将它与我相同的集合结构一起使用.

Any indexing methods is available there? so that I can use it along with my same collection structure.

提前致谢.

推荐答案

目前每个employee_details都会进行330000次的查找，但是如果我们在查找之前先进行排序和限制，那么只会是10次.这将大大减少查询时间.

Currently lookup will be made for every employee_details which means for 330000 times, but if we first sort and limit before lookup, it will be only 10 times. This will greatly decrease query time.

db.getCollection('employee_details').aggregate([
    {$sort      : {employee_fname: -1}},
    {$limit     :10},
    {
        $lookup : {
            from         : "departments",
            localField   : "department_id",
            foreignField : "_id",
            as           : "Department"
        }
    },
    { $unwind   : { path: "$Department", preserveNullAndEmptyArrays: true }},
]) 

尝试之后，如果您甚至想减少响应时间，您可以定义一个索引 在排序字段上.

After trying this, if you even want to decrease the response time you can define an index on the sort field.

db.employee_details.createIndex( { employee_fname: -1 } )

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