MongoDB 聚合以及使用关联集合字段进行查找和排序会减慢查询速度

Question

我的 mongodb 数据库中有两个集合，如下所示:

employee_details with approximately 330000 documents which has department_id as a reference from departments collection

departments collections with 2 fields _id and dept_name

我已将索引添加到集合中，如下所示

db.departments.createIndex( { dept_name: 1 } )
db.employee_details.createIndex( { department_id: 1 } )
db.employee_details.createIndex( { employee_fname: 1 } )

我想通过连接两个集合来获取要在数据表上列出的数据。但当我尝试这样做时，我遇到了两个问题。

首先，当我在查找后添加排序时，查询运行需要很长时间，我在查找后添加了排序，因为我需要使用集合部门的 dept_name 进行排序。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: { "Department.dept_name": 1 } },
  { $limit: 30 }
]);

其次，当我在查找上方添加排序时，查询变得很快，但如果我使用 dept_name 或 Department_id 排序，结果会给出错误的排序(排序对于 employee_details 集合的字段效果很好)。查询如下

db.getCollection("employee_details").aggregate([
  { $unwind: { path: "$Department", preserveNullAndEmptyArrays: true } },
  { $sort: { "Department.dept_name": 1 } },
  //{ $sort: { "department_id": 1 } }, // tried this also
  { $limit: 30 },
  {
    $lookup: {
      from: "departments",
      localField: "department_id",
      foreignField: "_id",
      as: "Department"
    }
  }
]);

有人可以提供一个优化的解决方案来从所有相关集合中获取数据并进行排序吗？ 预先感谢您。

Answer 1

试试这个:

db.departments.aggregate([
  {
    $sort: {
      "dept_name": 1
    }
  },
  {
    $lookup: {
      from: "employee_details",
      localField: "_id",
      foreignField: "department_id",
      as: "Employee"
    }
  },
  {
    $unwind: "$Employee"
  },
  {
    $addFields: {
      tmp: {
        $mergeObjects: [
          {
            Department: "$$ROOT"
          },
          "$Employee"
        ]
      }
    }
  },
  {
    $project: {
      "tmp.Department.Employee": 0
    }
  },
  {
    $addFields: {
      "tmp.Department": [
        "$tmp.Department"
      ]
    }
  },
  {
    $replaceRoot: {
      newRoot: "$tmp"
    }
  },
  {
    $limit: 30
  }
])

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