MySQL常用索引

1.主键索引

• 主键索引必定是唯一索引，且不允许空值

create table `example1` (
    `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '自增id',
    PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='示例1';

---

2.唯一索引

• 除开取值唯一以外，与普通索引没有区别

create table `example2` (
    `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '自增id',
    `student_id` int unsigned NOT NULL DEFAULT 0 COMMENT '学号',
    PRIMARY KEY (`id`),
    UNIQUE KEY `uniq_sid` (`student_id`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='示例2';

---

3.普通索引

• 建立索引应在区分度大的字段上，不要在低基数列上建立索引，比如 性别

create table `example3` (
    `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '自增id',
    `student_id` int unsigned NOT NULL DEFAULT 0 COMMENT '学号',
    `grade` int unsigned NOT NULL DEFAULT 0 COMMENT '分数',
    PRIMARY KEY (`id`),
    KEY `idx_grade` (`grade`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='示例3';

---

4.组合索引

• 区分度大的列放在前，有范围性的列放在后
• 联合索引(a, b, c)相当于(a), (a, b ), (a, b, c)
• 合理使用索引覆盖减少IO，避免排序
• 单个索引字段不应超过5个

create table `example3` (
    `id` int unsigned NOT NULL AUTO_INCREMENT COMMENT '自增id',
    `student_id` int unsigned NOT NULL DEFAULT 0 COMMENT '学号',
    `grade` int unsigned NOT NULL DEFAULT 0 COMMENT '分数',
    `create_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT '创建时间',
    PRIMARY KEY (`id`),
    KEY `idx_sid_grede_ctime` (`student_id`, `grade`, `create_time`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='示例3';