mongodb 的查詢如何使用索引

時間 2019-11-12

原文原文鏈接

1、準備
php

須要在mongodb中添加測試數據java

db.user.insert({"name":"張三","age":34,"job":"java 開發","phone":"18989834028","addr":"廣東深圳福田"})
db.user.insert({"name":"李四","age":29,"job":"php 開發","phone":"18984834098","addr":"廣東深圳南山"})
db.user.insert({"name":"王五","age":38,"job":"部門經理","phone":"18981834098","addr":"廣東深圳羅湖"})
db.user.insert({"name":"趙一","age":31,"job":"產品經理","phone":"18989764098","addr":"廣東廣州"})
db.user.insert({"name":"錢二","age":26,"job":"銷售","phone":"18989834968","addr":"廣東珠海"})
db.user.insert({"name":"週六","age":54,"job":"銷售經理","phone":"18981234098","addr":"廣東中山"})
db.user.insert({"name":"吳奇","age":17,"job":"學生","phone":"18735834098","addr":"廣東韶關"})
db.user.insert({"name":"鄭八","age":45,"job":"教師","phone":"18989834098","addr":"廣東惠州"})
db.user.insert({"name":"馮峯","age":23,"job":"java 開發","phone":"13689834098","addr":"廣東廣州越秀"})

咱們知道使用mongodb objectId數據主鍵_id是默認有索引的。下面先按名稱和手機建立單獨的索引，而後建立一個聯合主鍵mongodb

db.user.ensureIndex({"phone":1});
db.user.ensureIndex({"name":1});
db.user.ensureIndex({"name":1,"phone":1});

查看下如今已有的索引測試

>db.user.getIndexes();
[
        {
                "v" : 1,
                "key" : {
                        "_id" : 1
                },
                "name" : "_id_",
                "ns" : "test.user"
        },
        {
                "v" : 1,
                "key" : {
                        "name" : 1
                },
                "name" : "name_1",
                "ns" : "test.user"
        },
        {
                "v" : 1,
                "key" : {
                        "phone" : 1
                },
                "name" : "phone_1",
                "ns" : "test.user"
        },
        {
                "v" : 1,
                "key" : {
                        "name" : 1,
                        "phone" : 1
                },
                "name" : "name_1_phone_1",
                "ns" : "test.user"
        }
]

2、測試spa

使用explain()方法查看執行計劃，首先看下不使用查詢條件。code

> db.user.find().explain();
{
        "queryPlanner" : {
                  "winningPlan" : {
                        "stage" : "COLLSCAN",
  
}

能夠看到沒有使用查詢條件的狀況下使用的是全表掃描。索引

再來看下根據name,和phone做爲查詢條件的狀況下，使用的是聯合索引，仍是單獨的索引。開發

> db.user.find({"phone":"13678763456","name":"李斯"}).explain();
{
        "queryPlanner" : {
                  "winningPlan" : {
                                "indexName" : "name_1",                           
                },
                "rejectedPlans" : [
                     {
                                        "indexName" : "name_1_phone_1",
                                        ....                                      
                                }
                        },
                        {
.....
                                        "indexName" : "phone_1",
                                       ......                                       
                                }
                        }
                ]
        },
}

能夠看到，並無使用聯合主鍵並無在最優計劃（winningPlan）中，而是在rejectedplan中。先使用了phone的單獨索引。調整查詢條件順序，索引的順序是name,phone，看下get

> db.user.find({"name":"李斯","phone":"18737260965"}).explain();
{
        "queryPlanner" : {
            ....
                "winningPlan" : {
                       ......
                        "inputStage" : {
                               .......
                                "indexName" : "name_1",
                             ......                            
                        }
                },
                "rejectedPlans" : [
                        {
                                "inputStage" : {
                                      .........
                                        "indexName" : "name_1_phone_1",
                                      ........
                                     
                        },
                        {    
                                "inputStage" : {                                      
                                        "indexName" : "phone_1",
                                      .......                                    
                        }
                ]
        },

}

調整順序以後仍是同樣，在winningPlan使用name的單獨索引而不是聯合索引。刪除單獨的索引再測試input

db.user.dropIndex("name_1");
db.user.dropIndex("phone_1");

測試

> db.user.find({"name":"李斯","phone":"18737260965"}).explain();
....
 "winningPlan" : {                       
                       ...
                                "indexName" : "name_1_phone_1",
                                ...
                                
                 
> db.user.find({"phone":"13678763456","name":"李斯"}).explain();
  "winningPlan" : {
                       .....
                                "indexName" : "name_1_phone_1",
                                ....
> db.user.find({"phone":"13678763456","name":"李斯","age":23}).explain();
  "winningPlan" : {
  。。。
 "indexName" : "name_1_phone_1",
 
> db.user.find({"phone":"13678763456","age":23,"name":"李斯"}).explain();

 "winningPlan" : {
      "indexName" : "name_1_phone_1",
      
> db.user.find({"phone":"13678763456"}).explain();
 "winningPlan" : {
                        "stage" : "COLLSCAN",
                        
> db.user.find({"name":"張三"}).explain();
  "winningPlan" : {
。。。。
                                "indexName" : "name_1_phone_1",
                                
> db.user.find({"age":33,"name":"張三"}).explain();
 "winningPlan" : {
                     。。。。
                                "indexName" : "name_1_phone_1",

能夠看到對於聯合索引，若是隻使用一部分，則必須是按順序的（在本測試中只用使用name纔用到索引），若是知足條件（name,phone 都出如今條件中），則沒有順序要求。

3、終結

經過簡單的測試，對mongodb（ "version" : "3.2.4",）的索引工做原理理解：

一、若是某個字段同時出如今單獨索引和聯合索引中，winningPlan 使用的是單獨索引。

二、若是查詢條件知足索引的條件，則不須要管理條件字段順序，

三、若是查詢條件字段不知足聯合索引，則必須是按建立索引的字段順序查詢纔會使用索引。（能夠有其餘字段在索引字段前如，{"age":33,"name":"張三"}).