Example demonstration of MySQL index failure scenario

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In this paper, various effective situations of MySQL index failure, write sample SQL for each situation and view the execution plan in the database.

1. Environmental information

CentOS 7.4

Mysql 5.7.32

Two, table, data preparation

Create a user table

CREATE DATABASE IF NOT EXISTS `test` DEFAULT CHARACTER SET utf8;
USE `test`;

DROP TABLE IF EXISTS `t_user`;
CREATE TABLE `t_user`  (
  `id` bigint NOT NULL DEFAULT 0 COMMENT '主键，用户唯一id',
  `user_name` varchar(32) NOT NULL DEFAULT '' COMMENT '用户名',
  `password` varchar(64) NOT NULL DEFAULT '' COMMENT '密码',
  `email` varchar(32) NOT NULL DEFAULT '' COMMENT '邮箱',
  `phone_number` varchar(16) NOT NULL DEFAULT '' COMMENT '电话号码',
  `avatar` varchar(256) NOT NULL DEFAULT '' COMMENT '头像',
  `create_time` datetime NOT NULL ON UPDATE CURRENT_TIMESTAMP COMMENT '用户账号创建时间',
  `update_time` datetime NOT NULL ON UPDATE CURRENT_TIMESTAMP COMMENT '上次更新记录时间',
  `last_login_time` datetime NOT NULL ON UPDATE CURRENT_TIMESTAMP COMMENT '上次登录时间',
  `status` int(2) NOT NULL DEFAULT 0 COMMENT '用户状态 0-正常 1-封禁',
  PRIMARY KEY (`id`)
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_bin COMMENT = '用户信息表';

Create a stored procedure and insert 100,000 pieces of test data

DROP PROCEDURE if exists insert_t_user_test;

DELIMITER $$
CREATE PROCEDURE insert_t_user_test(IN loop_times INT)
BEGIN
    DECLARE var INT DEFAULT 0;
    WHILE var < loop_times DO
        SET var = var + 1;
    	  INSERT INTO `t_user` VALUES (var, CONCAT('rkyao-', var), '123456', '[email protected]', '15251831704', 'avatar.jpg', CURRENT_TIMESTAMP, CURRENT_TIMESTAMP, CURRENT_TIMESTAMP, 0);
    END WHILE;
		COMMIT;
END $$


CALL insert_t_user_test(100000);

Three, the failure of the composite index

Create composite index

CREATE INDEX `idx_user` ON `t_user` (`user_name`, `email`, `phone_number`);

leftmost prefix rule

Failure to comply with the leftmost prefix rule will invalidate the index, and the query must start from the leftmost column of the index without skipping the middle column, regardless of the condition order specified by where.

-- where查询字段顺序和索引定义一致
-- 走索引 命中三个字段 user_name, email, phone_number
explain select * from t_user where user_name = 'rkyao-1' and email = '[email protected]' and phone_number = '15251831704';

-- where查询字段顺序和索引定义不一致
-- 走索引 命中三个字段 user_name, email, phone_number
explain select * from t_user where phone_number = '15251831704' and email = '[email protected]' and user_name = 'rkyao-1';

-- 走索引 命中两个字段 user_name, email
explain select * from t_user where user_name = 'rkyao-1' and email = '[email protected]';

-- 走索引 命中一个字段 user_name
explain select * from t_user where user_name = 'rkyao-1' and phone_number = '15251831704';

-- where查询字段不包含索引最左边的列
-- 不走索引
explain select * from t_user where email = '[email protected]' and phone_number = '15251831704';

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or query

The or query will invalidate the index.

-- 不走索引
explain select * from t_user where user_name = 'rkyao-1' or email = '[email protected]';

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Fourth, the failure of single-column index

Create a single-column index on the and user_namefields separatelyemail

-- 删除联合索引
DROP INDEX `idx_user` ON `t_user`;
-- 创建单列索引
CREATE INDEX `idx_user_name` ON `t_user` (`user_name`);
CREATE INDEX `idx_user_email` ON `t_user` (`email`);

Like fuzzy query uses front wildcards

The use of wildcard matching before fuzzy query will invalidate the index.

-- 不走索引
explain select * from t_user where user_name like '%555%';
-- 不走索引
explain select * from t_user where user_name like '%555';
-- 走索引
explain select * from t_user where user_name like '555%';
-- 走索引
explain select * from t_user where user_name like '555';

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Using functions on indexed columns

Use functions on index columns, or perform operations without indexing

-- 不走索引
explain select * from t_user where concat(user_name, '123') = '555';

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String index without quotes

When querying a field of string type, the condition value is not quoted, and the index is not used

-- 不走索引
explain select * from t_user where user_name = 555;

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Use != , <>, >, <

Use not equal to, do not walk the index.

-- 不走索引
explain select * from t_user where user_name != '555';
-- 不走索引
explain select * from t_user where user_name <> '555';
-- 不走索引
explain select * from t_user where user_name > '555';

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is null 或 is not null

is null or is not null, no indexing.

-- 不走索引
explain select * from t_user where user_name is null;
-- 不走索引
explain select * from t_user where user_name is not null;

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in 或 not in

In goes to the index, use not in to not go to the index.

-- 走索引
explain select * from t_user where user_name in ('rkyao-6222', 'rkyao-5678');
-- 不走索引
explain select * from t_user where user_name not in ('rkyao-6222', 'rkyao-5678');

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Two single-column indexes and queries

When two index columns are queried by and in the where condition, only one index will take effect, that is, the index created earlier will take effect.

-- 走索引 命中user_name字段
explain select * from t_user where email = '[email protected]' and user_name = '555';

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Two single-column index or queries

When two index columns are queried by or in the where condition, both indexes will take effect.

-- 走索引 命中user_name字段
explain select * from t_user where email = '[email protected]' or user_name = '555';

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