MySql索引失效(版本8.0.18)

一、MySql的索引

1、索引失效問題

（1）總結

MySql在版本8.0.18中，導致索引失效的原因有

• 模糊查詢時，前模糊導致索引失效

• 對列使用函數操作，導致索引失效

• 使用<>、!=導致索引失效

• 當字段是字符串類型，傳入數字類型導致索引失效

• 複合索引時，條件1失效或者是不存在，導致索引失效

MySql在版本5.6中，導致索引失效的原因額外增加了幾種

• 使用 IN 導致索引失效

• 使用 OR 導致索引失效

（2）普通索引

1. 新建一張測試表

   CREATE TABLE `Test` (
     `id` bigint(20) NOT NULL AUTO_INCREMENT,
     `column1` varchar(128) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `column2` timestamp NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創建時間',
     `column3` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `column4` varchar(10000) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `column5` varchar(64) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `varcharKey` varchar(64) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `bigintKey` bigint(20) NOT NULL,
     PRIMARY KEY (`id`),
     KEY `idx_varcharKey` (`varcharKey`) USING BTREE,
     KEY `idx_bigintKey` (`bigintKey`) USING BTREE
   ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

    

2. 使用or查詢並未導致索引失效？

   #使用or查詢並未導致索引失效？
   EXPLAIN
   SELECT * FROM Test WHERE varcharKey = '2019-12-21' or varcharKey = '2019-12-22';

3. 使用模糊前綴，索引失效

   #使用模糊前綴，索引失效
   EXPLAIN
   SELECT * FROM Test WHERE varcharKey like '%2019-12-21';

4. 使用函數操作列，索引失效

   #使用函數操作，索引失效
   EXPLAIN
   SELECT * FROM Test WHERE bigintKey + 1 = 64967499;

5. 使用<>、!=操作，索引失效

   #使用<>、!=操作，索引失效
   EXPLAIN
   SELECT *     FROM Test WHERE bigintKey <> 64967499;
   EXPLAIN
   SELECT *     FROM Test WHERE bigintKey != 64967499;

6. 當數據庫字段是字符類型，傳入數字，索引失效
   當數據庫字段是數字，傳入字符，索引未失效

   #當數據庫字段是字符類型，傳入數字，索引失效
   #當數據庫字段是數字，傳入字符，索引未失效
   EXPLAIN
   SELECT *     FROM Test WHERE varcharKey = 64967499;
   EXPLAIN
   SELECT *     FROM Test WHERE bigintKey = '64967499';

（3）複合索引

1. 新建一張測試表

   CREATE TABLE `Test2` (
     `id` bigint(20) NOT NULL AUTO_INCREMENT,
     `column1` timestamp NULL DEFAULT CURRENT_TIMESTAMP COMMENT '創建時間',
     `column2` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `column3` varchar(10000) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `column4` varchar(64) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `varcharKey` varchar(128) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `varcharKey2` varchar(64) CHARACTER SET utf8mb4 COLLATE utf8mb4_0900_ai_ci DEFAULT NULL,
     `bigintKey` bigint(20) NOT NULL,
     PRIMARY KEY (`id`),
     KEY `idx_varcharKey1_varcharKey2_bigintKey` (`varcharKey`,`varcharKey2`,`bigintKey`) USING BTREE
   ) ENGINE=InnoDB AUTO_INCREMENT=233168 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

    

2. 缺少條件1不會走索引

   #缺少條件1不會走索引
   EXPLAIN
   SELECT *     FROM Test2 WHERE varcharKey = '64967499';
   EXPLAIN
   SELECT *     FROM Test2 WHERE varcharKey2 = '64967499';
   EXPLAIN
   SELECT *     FROM Test2 WHERE bigintKey = 64967499;

3. 條件1索引失效，不會走索引

   #條件1索引失效，不會走索引
   EXPLAIN
   SELECT * 	FROM Test2 WHERE varcharKey <> '64967499' and varcharKey2 = '64967499' and bigintKey = 64967499;
   EXPLAIN
   SELECT * 	FROM Test2 WHERE varcharKey = '64967499' and varcharKey2 <> '64967499' and bigintKey = 64967499;
   EXPLAIN
   SELECT * 	FROM Test2 WHERE varcharKey = '64967499' and varcharKey2 = '64967499' and bigintKey <> 64967499;

二、 Mysql版本5.6

1、索引失效

（1）使用 OR 導致索引失效

EXPLAIN
SELECT * FROM Test WHERE varcharKey = '2019-12-21' or varcharKey = '2019-12-22';

（2）使用 IN 導致索引失效

EXPLAIN
SELECT * FROM test WHERE bigintKey IN (1,4);