mysql 的索引下推（IndexCondition）简单理解

前置版本

mysql> select version();
+------------+
| version()  |
+------------+
| 5.7.26-log |
+------------+

先来建个表加个索引

DROP TABLE IF EXISTS `indexconditionpushdown`;
CREATE TABLE `indexconditionpushdown` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `age` int(11) NOT NULL,
  `name` varchar(255) COLLATE utf8_unicode_ci NOT NULL,
  PRIMARY KEY (`id`),
  KEY `multi_name_age` (`name`,`age`)
) ENGINE=MyISAM AUTO_INCREMENT=4 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;

-- ----------------------------
-- Records of indexconditionpushdown
-- ----------------------------
INSERT INTO `indexconditionpushdown` VALUES ('1', '20', '陈某');
INSERT INTO `indexconditionpushdown` VALUES ('2', '30', '陈某某');
INSERT INTO `indexconditionpushdown` VALUES ('3', '30', '李某');

alter table indexConditionPushDown add index multi_name_age(name,age);

ICP版本

mysql> explain select * from indexConditionPushDown where name like "陈%" and age =20;
+----+-------------+------------------------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+
| id | select_type | table                  | partitions | type  | possible_keys  | key            | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+------------------------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | indexConditionPushDown | NULL       | range | multi_name_age | multi_name_age | 771     | NULL |    1 |    33.33 | Using index condition |
+----+-------------+------------------------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+
1 row in set

禁用ICP

mysql> set optimizer_switch='index_condition_pushdown=off';
Query OK, 0 rows affected

mysql> explain select * from indexConditionPushDown where name like "陈%" and age =20;
+----+-------------+------------------------+------------+-------+----------------+----------------+---------+------+------+----------+-------------+
| id | select_type | table                  | partitions | type  | possible_keys  | key            | key_len | ref  | rows | filtered | Extra       |
+----+-------------+------------------------+------------+-------+----------------+----------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | indexConditionPushDown | NULL       | range | multi_name_age | multi_name_age | 771     | NULL |    1 |    33.33 | Using where |
+----+-------------+------------------------+------------+-------+----------------+----------------+---------+------+------+----------+-------------+
1 row in set

结论 【Extra】

1. ICP版本为 Using index condition
2. 禁用后未 Using where