MySQL回表,覆盖索引,索引下推

SELECT * FROM INNODB_USER
WHERE AGE = 18 AND USER_NAME LIKE '模糊查%';

DROP TABLE IF EXISTS `t_user`;
CREATE TABLE `t_user` (
  `id` bigint NOT NULL AUTO_INCREMENT COMMENT '主键',
  `name` varchar(64) NOT NULL COMMENT '用户名 ',
  `age` int(8) NOT NULL COMMENT '年纪',
	`address` varchar(255) DEFAULT NULL COMMENT '地址',
  `is_delete` tinyint NOT NULL DEFAULT '0' COMMENT '是否删除 默认否',
  `create_time` datetime DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
  PRIMARY KEY (`id`),
  KEY `idx_name_age` (`name`,`age`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci COMMENT='用户信息表';

-- 初始化数据 
INSERT INTO `t_user`(`name`, `age`, `address`) VALUES ('李四', 22, '中国');
INSERT INTO `t_user`(`name`, `age`, `address`) VALUES ('李五', 22, '中国');
INSERT INTO `t_user`(`name`, `age`, `address`) VALUES ('李六', 23, '中国');
INSERT INTO `t_user`(`name`, `age`, `address`) VALUES ('张三', 24, '中国');
INSERT INTO `t_user`(`name`, `age`, `address`) VALUES ('李期', 24, '中国');

show VARIABLES like '%optimizer_switch%';
-------------------------------------------------------
optimizer_switch	index_merge=on,index_merge_uNIOn=on,index_merge_sort_union=on,index_merge_intersection=on,engine_condition_pushdown=on,index_condition_pushdown=on,mrr=on,mrr_cost_based=on,block_nested_loop=on,batched_key_access=off,materialization=on,semijoin=on,loosescan=on,firstmatch=on,duplicateweedout=on,subquery_materialization_cost_based=on,use_index_extensions=on,condition_fanout_filter=on,derived_merge=on,use_invisible_indexes=off,skip_scan=on,hash_join=on,subquery_to_derived=off,prefer_ordering_index=on,hypergraph_optimizer=off,derived_condition_pushdown=on

索引下推是mysql 5.6优化查询回表的功能，在5.6之前都不支持索引下推，笔者用的8.0，则需要先关闭索引下推：
set optimizer_switch='index_condition_pushdown=off';

EXPLAIN SELECT * from t_user where name like '李%' and age = 22;

set optimizer_switch='index_condition_pushdown=on';

EXPLAIN SELECT * from t_user where name like '李%' and age = 22;