MySQL查询条件中in会用到索引吗

当用人问你MySQL 查询条件中 in 会不会用到索引，你该怎么回答？

答案：可能会用到索引

动手来测试下

1.创建一张表，给字段port建立索引

CREATE TABLE `pre_request_logs_20180524` ( `id` int(11) NOT NULL AUTO_INCREMENT, `ip` char(16) NOT NULL COMMENT '代理IP', `port` int(8) NOT NULL COMMENT '端口号', `status` enum('成功','失败') NOT NULL COMMENT '状态', `create_time` datetime NOT NULL COMMENT '创建时间', `update_time` datetime DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP, PRIMARY KEY (`id`), KEY `idx_port` (`port`) USING BTREE) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=utf8 COMMENT='代理IP请求日志';

INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (1, '192.168.1.199', 53149, '失败', '2018-05-24 14:55:34', '2018-11-16 10:58:13');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (2, '192.168.1.100', 10653, '成功', '2018-05-24 14:55:54', '2018-11-16 10:58:13');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (3, '192.168.1.112', 50359, '失败', '2018-05-24 14:56:00', '2018-11-16 10:58:13');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (4, '192.168.1.67', 30426, '失败', '2018-05-24 14:56:09', '2018-11-16 10:58:13');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (5, '192.168.1.209', 49323, '失败', '2018-05-24 14:56:12', '2018-11-16 10:58:13');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (6, '192.168.1.209', 51161, '成功', '2018-05-24 14:56:13', '2018-11-16 10:58:13');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (7, '192.168.1.12', 54167, '成功', '2018-05-24 14:56:16', '2018-11-16 10:58:13');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (8, '192.168.1.64', 20462, '成功', '2018-05-24 14:56:19', '2018-11-16 10:58:13');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (9, '192.168.1.53', 22823, '失败', '2018-05-24 14:56:31', '2018-11-16 10:58:13');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (10, '192.168.1.85', 48229, '成功', '2018-05-24 14:56:32', '2018-11-16 11:01:11');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (11, '192.168.1.85', 48229, '成功', '2018-05-24 14:56:32', '2018-11-16 11:01:15');INSERT INTO ``(`id`, `ip`, `port`, `status`, `create_time`, `update_time`) VALUES (12, '192.168.1.85', 48229, '成功', '2018-05-24 14:56:32', '2018-11-16 13:34:37');

explain select * from pre_request_logs_20180524 where port in (51161,20462,48229);

从结果来看是没有用到索引，但不要着急下结论，再看二个sql

select * from pre_request_logs_20180524 where port in (51161,48229);select * from pre_request_logs_20180524 where port in (51161,20462);

可以看到第二条sql是用到了索引，二条sql的区别在于port值不一样，一个包含48229，一个包含20462

其实MySQL优化器会自动判断in是否走二级索引，也就是port字段的索引

总结

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