Mysql索引选择以及优化详解

// 假设一个数据页16K、一行数据1K、索引间指针6字节、索引字段bigint类型(8字节)

// 索引个数
K = 16*1024/(8+6) =1170

// 单个叶子节点记录数
N = 16/1 = 16

// 三层B+记录数
V = K*K*N = 21902400

-- 查看当前索引基数
Mysql> show index from test;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test |   0 | PRIMARY |   1 | id   | A   |  100256 | NULL  | NULL |  | BTREE  |   |    |
| test |   1 | index_a |   1 | a   | A   |  98199 | NULL  | NULL | YES | BTREE  |   |    |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

mysql> show variables like '%innodb_stats_persistent%';
+--------------------------------------+-------------+
| Variable_name      | Value  |
+--------------------------------------+-------------+
-- 是否自动触发更新统计信息，当被修改的数据超过10%时就会触发统计信息重新统计计算
| innodb_stats_auto_recalc    | ON   |
-- 控制在重新计算统计信息时是否会考虑删除标记的记录
| innodb_stats_include_delete_marked | OFF   |
-- 对null值的统计方法，当变量设置为nulls_equal时，所有NULL值都被视为相同
| innodb_stats_method     | nulls_equal | 
-- 操作元数据时是否触发更新统计信息
| innodb_stats_on_metadata    | OFF   |
-- 统计信息是否持久化存储
| innodb_stats_persistent    | ON   |
-- innodb_stats_persistent=on，持久化统计信息采样的抽样页数
| innodb_stats_persistent_sample_pages | 20   |
-- 不推荐使用，已经被innodb_stats_transient_sample_pages替换
| innodb_stats_sample_pages   | 8   |
-- 瞬时抽样page数
| innodb_stats_transient_sample_pages | 8   |
+--------------------------------------+-------------+

-- 重新统计索引信息
mysql> analyze table t;

-- 创建表
mysql> CREATE TABLE `t` (
`id` int(11) NOT NULL,
`a` int(11) DEFAULT NULL,
`b` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `a` (`a`),
KEY `b` (`b`)
) ENGINE=InnoDB;

-- 定义测试数据存储过程
mysql> delimiter ;
CREATE PROCEDURE idata ()
BEGIN

DECLARE i INT ;
SET i = 1 ;
WHILE (i <= 100000) DO
 INSERT INTO t
VALUES
 (i, i, i) ;
SET i = i + 1 ;
END
WHILE ;
END;
delimiter ;

-- 执行存储过程，插入测试数据
mysql> CALL idata ();

-- 查看执行计划，使用了字段a上的索引
mysql> explain select * from t where a between 10000 and 20000;
+----+-------------+-------+-------+---------------+-----+---------+------+-------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra         |
+----+-------------+-------+-------+---------------+-----+---------+------+-------+-----------------------+
| 1 | SIMPLE   | t   | range | a       | a  | 5    | NULL | 10000 | Using index condition |
+----+-------------+-------+-------+---------------+-----+---------+------+-------+-----------------------+

-- 由于需要进行字段b排序，虽然索引b需要扫描更多的行数，但本身是有序的，综合扫描行数和排序，优化器选择了索引b，认为代价更小
mysql> explain select * from t where (a between 1 and 1000) and (b between 50000 and 100000) order by b limit 1;
+----+-------------+-------+-------+---------------+-----+---------+------+-------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra               |
+----+-------------+-------+-------+---------------+-----+---------+------+-------+------------------------------------+
| 1 | SIMPLE   | t   | range | a,b      | b  | 5    | NULL | 50128 | Using index condition; Using where |
+----+-------------+-------+-------+---------------+-----+---------+------+-------+------------------------------------+

-- 方案1：通过force index强制走索引a，纠正优化器错误的选择，不建议使用（不通用，且索引名称更变语句也需要变）
mysql> explain select * from t force index(a) where (a between 1 and 1000) and (b between 50000 and 100000) order by b limit 1;
+----+-------------+-------+-------+---------------+-----+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra                       |
+----+-------------+-------+-------+---------------+-----+---------+------+------+----------------------------------------------------+
| 1 | SIMPLE   | t   | range | a       | a  | 5    | NULL | 999 | Using index condition; Using where; Using filesort |
+----+-------------+-------+-------+---------------+-----+---------+------+------+----------------------------------------------------+

-- 方案2：引导 MySQL 使用我们期望的索引，按b,a排序，优化器需要考虑a排序的代价
mysql> explain select * from t where (a between 1 and 1000) and (b between 50000 and 100000) order by b,a limit 1;
+----+-------------+-------+-------+---------------+-----+---------+------+------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra                       |
+----+-------------+-------+-------+---------------+-----+---------+------+------+----------------------------------------------------+
| 1 | SIMPLE   | t   | range | a,b      | a  | 5    | NULL | 999 | Using index condition; Using where; Using filesort |
+----+-------------+-------+-------+---------------+-----+---------+------+------+----------------------------------------------------+

-- 方案3：有些场景下，我们可以新建一个更合适的索引，来提供给优化器做选择，或删掉误用的索引
ALTER TABLE `t`
DROP INDEX `a`,
DROP INDEX `b`,
ADD INDEX `ab` (`a`,`b`) ;

-- 表t中字段xxx的索引选择性
select count(distinct xxx)/count(id) from t;

-- 只需要查 ID 的值，而 ID 的值已经在 k 索引树上了，因此可以直接提供查询结果，不需要回表
select ID from T where k between 3 and 5

-- 增加字段V，每次查询需要返回V，可考虑把k、v做成联合索引
select ID,V from T where k between 3 and 5

-- id、name、age三列，name、age上创建联合索引

-- 满足最左前缀原则，name、age均走索引
select * from T where name='xxx' and age=12

-- Mysql自动优化，调整name、age顺序，，name、age均走索引
select * from T where age=12 and name='xxx'

-- name满足最左前缀原则走索引，MySQL5.6引入索引下推优化（index condition pushdown)，即索引中先过滤掉不满足age=12的记录再回表
select * from T where name like 'xxx%' and age=12

-- 不满足最左前缀原则，均不走索引
select * from T where name like '%xxx%' and age=12

-- 满足最左前缀原则，name走索引
select * from T where name='xxx'

-- 不满足最左前缀原则，不走索引
select * from T where age=12

mysql> create table SUser(
ID bigint unsigned primary key,
name varchar(64),  
email varchar(64),
...
)engine=innodb;

-- 以下查询场景
mysql> select name from SUser where email='xxx';

-- 方案1：全文本索引，回表次数由符合条件的数据量决定
mysql> alter table SUser add index index1(email);

-- 方案2：前缀索引，回表次数由前缀匹配结果决定
mysql> alter table SUser add index index2(email(6));

-- 预设一个可以接受的区分度损失比，选择满足条件中最小的前缀长度
select count(distinct left(email,n))/count(distinct email) from SUser;

-- 查询时字符串反转查询
mysql> select field_list from t where id_card = reverse('input_id_card_string');

-- 创建一个整数字段，来保存身份证的校验码，同时在这个字段上创建索引
mysql> alter table t add id_card_crc int unsigned, add index(id_card_crc);

-- 查询时使用hash字段走索引查询，再使用原字段精度过滤
mysql> select field_list from t where id_card_crc=crc32('input_id_card_string') and id_card='input_id_card_string'

-- 使用前缀索引就用不上覆盖索引对查询性能的优化
select id,email from SUser where email='xxx';