MySQL中覆盖索引怎么用

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查看测试表结构：

mysql> show create table im_message \G*************************** 1. row ***************************       Table: im_messageCreate Table: CREATE TABLE `im_message` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `from_id` varchar(40) COLLATE utf8_bin NOT NULL,  `from_type` tinyint(1) NOT NULL DEFAULT '0',  `to_id` varchar(40) COLLATE utf8_bin NOT NULL,  `to_type` tinyint(1) NOT NULL DEFAULT '0',  `content` varchar(2048) COLLATE utf8_bin DEFAULT '',  `create_date` bigint(20) NOT NULL,  `update_date` bigint(20) NOT NULL,  `message_id` varchar(40) COLLATE utf8_bin NOT NULL,  `is_sync` tinyint(1) DEFAULT '0' COMMENT '是否同步   0 未同步    1  已同步',  `is_read` tinyint(1) DEFAULT '0' COMMENT '是否已读   0 未读    1  已读',  `is_withdraw` tinyint(1) DEFAULT '0' COMMENT '是否撤回  0 未撤  1  已撤',  `is_lastest` tinyint(1) DEFAULT '0' COMMENT '是否是最新回话消息 0 不是 1是',  PRIMARY KEY (`id`),  UNIQUE KEY `uidx_message_id` (`message_id`),  KEY `idx_date` (`create_date`),  KEY `idx_from_id` (`from_id`),  KEY `idx_to_id` (`to_id`),  KEY `idx_is_sync` (`is_sync`),  KEY `idx_update_date` (`update_date`),  KEY `idx_fid_tid` (`from_id`,`to_id`)) ENGINE=InnoDB AUTO_INCREMENT=13264365 DEFAULT CHARSET=utf8 COLLATE=utf8_bin ROW_FORMAT=DYNAMIC1 row in set (0.00 sec)

查看SQL执行效果：

select max(id) as "id" from im_message WHERE from_id='bd29879279c44672a2fdffcb8428b4d1' or to_id='bd29879279c44672a2fdffcb8428b4d1' group by from_id, to_id；|  8547247 ||  7152294 ||  6897129 || 12874034 || 10011290 ||  8027198 ||  7852741 ||  9960496 ||  6059399 || 10860981 ||  9963172 || 13253445 |+----------+27827 rows in set (0.91 sec)

查看执行计划：

mysql> explain select max(id) as "id" from im_message WHERE from_id='bd29879279c44672a2fdffcb8428b4d1' or to_id='bd29879279c44672a2fdffcb8428b4d1' group by from_id, to_id;

+----+-------------+------------+------------+-------------+-----------------------------------+-----------------------+---------+------+--------+----------+---------------------------------------------------------------------------------------+

| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |

+----+-------------+------------+------------+-------------+-----------------------------------+-----------------------+---------+------+--------+----------+---------------------------------------------------------------------------------------+

| 1 | SIMPLE | im_message | NULL | index_merge | idx_from_id,idx_to_id,idx_fid_tid | idx_fid_tid,idx_to_id | 122,122 | NULL | 168680 | 100.00 | Using sort_union(idx_fid_tid,idx_to_id); Using where; Using temporary; Using filesort |

+----+-------------+------------+------------+-------------+-----------------------------------+-----------------------+---------+------+--------+----------+---------------------------------------------------------------------------------------+

1 row in set, 1 warning (0.00 sec)

创建覆盖索引：

mysql> alter table im_message add index idx_from_id_id(from_id,id);Query OK, 0 rows affected (1 min 1.94 sec)Records: 0  Duplicates: 0  Warnings: 0mysql> alter table im_message add index idx_to_id_id(to_id,id);Query OK, 0 rows affected (1 min 9.79 sec)Records: 0  Duplicates: 0  Warnings: 0

重新查看SQL执行效果：

select max(id) as "id" from im_message WHERE from_id='bd29879279c44672a2fdffcb8428b4d1' or to_id='bd29879279c44672a2fdffcb8428b4d1' group by from_id, to_id;|  8547247 ||  7152294 ||  6897129 || 12874034 || 10011290 ||  8027198 ||  7852741 ||  9960496 ||  6059399 || 10860981 ||  9963172 || 13253445 |+----------+27827 rows in set (0.63 sec)

查看执行计划：

mysql> explain select max(id) as "id" from im_message WHERE from_id='bd29879279c44672a2fdffcb8428b4d1' or to_id='bd29879279c44672a2fdffcb8428b4d1' group by from_id, to_id;+----+-------------+------------+------------+-------------+---------------------------------------------------------------+-----------------------------+---------+------+--------+----------+----------------------------------------------------------------------------------------+| id | select_type | table      | partitions | type        | possible_keys                                                 | key                         | key_len | ref  | rows   | filtered | Extra                                                                                  |+----+-------------+------------+------------+-------------+---------------------------------------------------------------+-----------------------------+---------+------+--------+----------+----------------------------------------------------------------------------------------+|  1 | SIMPLE      | im_message | NULL       | index_merge | idx_from_id,idx_to_id,idx_fid_tid,idx_from_id_id,idx_to_id_id | idx_from_id_id,idx_to_id_id | 122,122 | NULL | 162106 |   100.00 | Using union(idx_from_id_id,idx_to_id_id); Using where; Using temporary; Using filesort |+----+-------------+------------+------------+-------------+---------------------------------------------------------------+-----------------------------+---------+------+--------+----------+----------------------------------------------------------------------------------------+1 row in set, 1 warning (0.00 sec)

发现优化器选择了新建的两个覆盖索引。

创建覆盖索引之后，利用索引的有序性， select max(id)可以快速的取到最大id。

以上是"MySQL中覆盖索引怎么用"这篇文章的所有内容，感谢各位的阅读！相信大家都有了一定的了解，希望分享的内容对大家有所帮助，如果还想学习更多知识，欢迎关注行业资讯频道！