数据库内核月报

概述

MySQL从5.7.8起开始支持JSON字段，这极大的丰富了MySQL的数据类型。也方便了广大开发人员。但MySQL并没有提供对JSON对象中的字段进行索引的功能，至少没有直接对其字段进行索引的方法。本文将介绍利用MySQL 5.7中的虚拟字段的功能来对JSON对象中的字段进行索引。

示例数据

我们将基于下面的JSON对象进行演示

{
    "id": 1,  
    "name": "Sally",  
    "games_played":{    
       "Battlefield": {
          "weapon": "sniper rifle",
          "rank": "Sergeant V",
          "level": 20
        },                                                                                                                          
       "Crazy Tennis": {
          "won": 4,
          "lost": 1
        },  
       "Puzzler": {
          "time": 7
        }
     }
 }

表的基本结构

CREATE TABLE `players` (  
    `id` INT UNSIGNED NOT NULL,
    `player_and_games` JSON NOT NULL,
    PRIMARY KEY (`id`)
);

如果只是基于上面的表的结构我们是无法对JSON字段中的Key进行索引的。接下来我们演示如何借助虚拟字段对其进行索引

增加虚拟字段

虚拟列语法如下

<type> [ GENERATED ALWAYS ] AS ( <expression> ) [ VIRTUAL|STORED ]
[ UNIQUE [KEY] ] [ [PRIMARY] KEY ] [ NOT NULL ] [ COMMENT <text> ]

在MySQL 5.7中，支持两种Generated Column，即Virtual Generated Column和Stored Generated Column，前者只将Generated Column保存在数据字典中（表的元数据），并不会将这一列数据持久化到磁盘上；后者会将Generated Column持久化到磁盘上，而不是每次读取的时候计算所得。很明显，后者存放了可以通过已有数据计算而得的数据，需要更多的磁盘空间，与Virtual Column相比并没有优势，因此，MySQL 5.7中，不指定Generated Column的类型，默认是Virtual Column。

如果需要Stored Generated Golumn的话，可能在Virtual Generated Column上建立索引更加合适，一般情况下，都使用Virtual Generated Column，这也是MySQL默认的方式

加完虚拟列的建表语句如下：

CREATE TABLE `players` (  
   `id` INT UNSIGNED NOT NULL,
   `player_and_games` JSON NOT NULL,
   `names_virtual` VARCHAR(20) GENERATED ALWAYS AS (`player_and_games` ->> '$.name') NOT NULL, 
   PRIMARY KEY (`id`)
);

Note: 利用操作符-» 来引用JSON字段中的KEY。在本例中字段names_virtual为虚拟字段，我把它定义成不可以为空。在实际的工作中，一定要集合具体的情况来定。因为JSON本身是一种弱结构的数据对象。也就是说的它的结构不是固定不变的。

我们插入数据

INSERT INTO `players` (`id`, `player_and_games`) VALUES (1, '{  
    "id": 1,  
    "name": "Sally",
    "games_played":{    
       "Battlefield": {
          "weapon": "sniper rifle",
          "rank": "Sergeant V",
          "level": 20
        },                                                                                                                          
       "Crazy Tennis": {
          "won": 4,
          "lost": 1
        },  
       "Puzzler": {
          "time": 7
        }
      }
   }'
);
...

查看表里的数据

SELECT * FROM `players`;

+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------+
| id | player_and_games                                                                                                                                                                                           | names_virtual |
+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------+
|  1 | {"id": 1, "name": "Sally", "games_played": {"Puzzler": {"time": 7}, "Battlefield": {"rank": "Sergeant V", "level": 20, "weapon": "sniper rifle"}, "Crazy Tennis": {"won": 4, "lost": 1}}}                  | Sally         |
|  2 | {"id": 2, "name": "Thom", "games_played": {"Puzzler": {"time": 25}, "Battlefield": {"rank": "Major General VIII", "level": 127, "weapon": "carbine"}, "Crazy Tennis": {"won": 10, "lost": 30}}}            | Thom          |
|  3 | {"id": 3, "name": "Ali", "games_played": {"Puzzler": {"time": 12}, "Battlefield": {"rank": "First Sergeant II", "level": 37, "weapon": "machine gun"}, "Crazy Tennis": {"won": 30, "lost": 21}}}           | Ali           |
|  4 | {"id": 4, "name": "Alfred", "games_played": {"Puzzler": {"time": 10}, "Battlefield": {"rank": "Chief Warrant Officer Five III", "level": 73, "weapon": "pistol"}, "Crazy Tennis": {"won": 47, "lost": 2}}} | Alfred        |
|  5 | {"id": 5, "name": "Phil", "games_played": {"Puzzler": {"time": 7}, "Battlefield": {"rank": "Lt. Colonel III", "level": 98, "weapon": "assault rifle"}, "Crazy Tennis": {"won": 130, "lost": 75}}}          | Phil          |
|  6 | {"id": 6, "name": "Henry", "games_played": {"Puzzler": {"time": 17}, "Battlefield": {"rank": "Captain II", "level": 87, "weapon": "assault rifle"}, "Crazy Tennis": {"won": 68, "lost": 149}}}             | Henry         |
+----+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+---------------+

查看表Players的字段

SHOW COLUMNS FROM `players`;

+------------------+------------------+------+-----+---------+-------------------+
| Field            | Type             | Null | Key | Default | Extra             |
+------------------+------------------+------+-----+---------+-------------------+
| id               | int(10) unsigned | NO   | PRI | NULL    |                   |
| player_and_games | json             | NO   |     | NULL    |                   |
| names_virtual    | varchar(20)      | NO   |     | NULL    | VIRTUAL GENERATED |
+------------------+------------------+------+-----+---------+-------------------+

我们看到虚拟字段names_virtual的类型是VIRTUAL GENERATED。MySQL只是在数据字典里保存该字段元数据，并没有真正的存储该字段的值。这样表的大小并没有增加。我们可以利用索引把这个字段上的值进行物理存储。

在虚拟字段上加索引

再添加索引之前，让我们先看下面查询的执行计划

EXPLAIN SELECT * FROM `players` WHERE `names_virtual` = "Sally"\G  
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: players
   partitions: NULL
         type: ALL
possible_keys: NULL  
          key: NULL
      key_len: NULL
          ref: NULL
         rows: 6
     filtered: 16.67
        Extra: Using where

添加索引

CREATE INDEX `names_idx` ON `players`(`names_virtual`);

再执行上面的查询语句，我们将得到不一样的执行计划

EXPLAIN SELECT * FROM `players` WHERE `names_virtual` = "Sally"\G  
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: players
   partitions: NULL
         type: ref
possible_keys: names_idx  
          key: names_idx
      key_len: 22
          ref: const
         rows: 1
     filtered: 100.00
        Extra: NULL

如我们所见，最新的执行计划走了新建的索引。

小结

本文介绍了如何在MySQL 5.7中保存JSON文档。为了高效的检索JSON中内容，我们可以利用5.7的虚拟字段来对JSON的不同的KEY来建索引。极大的提高检索的速度。

数据库内核月报－ 2017 / 12

MySQL · 最佳实践 · 如何索引JSON字段

概述

示例数据

增加虚拟字段

在虚拟字段上加索引

小结