MySQL query slow even with indexes

Question

Structure of my table

mysql> show create table t_group_tag_relation\G
*************************** 1. row ***************************
Table: t_group_tag_relation
Create Table: CREATE TABLE `t_group_tag_relation` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`domain_id` int(11) NOT NULL,
`group_tag_id` int(11) NOT NULL,
`resource_id` int(11) NOT NULL,
`resource_type` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `group_tag_id_resource_id` (`group_tag_id`,`resource_id`),
KEY `domain_id` (`domain_id`,`group_tag_id`,`resource_type`,`resource_id`),
KEY `domain_id_resource_type` (`domain_id`,`resource_type`)
) ENGINE=InnoDB AUTO_INCREMENT=1613462 DEFAULT CHARSET=latin1
1 row in set (0.03 sec)

Explain plan of my query

mysql> explain select * from t_group_tag_relation WHERE resource_id = 575868070 AND domain_id = 476 AND resource_type = 2;
+----+-------------+----------------------+------+-----------------------------------+-----------+---------+-------+-------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+----------------------+------+-----------------------------------+-----------+---------+-------+-------+--------------------------+
| 1 | SIMPLE       | t_group_tag_relation | ref   | domain_id,domain_id_resource_type | domain_id | 4 | const | 28400 | Using where; Using index |
+----+-------------+----------------------+------+-----------------------------------+-----------+---------+-------+-------+--------------------------+
1 row in set (0.57 sec)

It is using indexes already but performance is not good. How to improve performance?

Answer 1

I think your explain plan is doing a heavy index scan. What you might need is an index with those exact three columns.

Please create this index

ALTER TABLE t_group_tag_relation ADD INDEX
resource_id_domain_id_resource_type_index (resource_id,domain_id,resource_type);

and try your query again.

Answer 2

Another test you can try... "probably" in your table the cardinality of 'domain_id_resource_type' index is lower than 'domain_id', you can try to skip the optimizer choice and declare the usage of 'domain_id_resource_type' instead of 'domain_id'

select * from t_group_tag_relation USE INDEX(domain_id_resource_type) WHERE resource_id = 575868070 AND domain_id = 476 AND resource_type = 2;