MySQL 8.0 倒序索引的应用
大家好，我是知数堂SQL 优化班老师 网名：骑龟的兔子
已经很久没写文章了，今天在浏览网站的时候，看到有人提了以下问题
为什么 下面的SQL 没有用到
index condition pushdown
提问者 判断到 下面的SQL 中 column4 的代价更高
所以创建了 idx1 ( column1,column2,column4,column3)
这样的索引，他认为 应该走icp 但是实际上，没有走
SELECT
t2.column1 ,
t3.column1
FROM tb_test1 t1
INNER JOIN tb_test2 t2 ON t2.column1 = t1.column1
INNER JOIN tb_test3 ON t3.column1 = t1.column1
WHERE
t1.coulmn1 = #{idNo}
AND t1.column2 = 'N'
AND t1.column3 in ( 'A','B','C')
ORDER BY t1.column4 DESC
LIMIT #{offset}, #{limit};
+----+-------------+-------+--------+---------------+---------+---------------------------------------+
| id | select_type | table | type | key | key_len | Extra |
+----+-------------+-------+--------+---------------+---------+---------------------------------------+
| 1 | SIMPLE | t1 | range | idx1_tb_test1 | 406 | Using where; Backward index scan |
| 1 | SIMPLE | t2 | eq_ref | PRIMARY | 8 | NULL |
| 1 | SIMPLE | t3 | eq_ref | PRIMARY | 8 | NULL |
+----+-------------+-------+--------+---------------+---------+---------------------------------------+
看到这里，突然我也想做个实验试一下
上我的课同学直接用下面搭建环境
create table t12 (
id bigint not null auto_increment primary key ,
emp_no int ,
salary int ,
from_date date ,
to_date date
)
insert into t12 (
emp_no
,salary
,from_date
,to_date
)
select
emp_no
,salary
,from_date
,to_date
from salaries limit 10000;
create index ix_t1 on t12(emp_no,from_date,salary);
然后运行了如下SQL
结果跟提问者一样的结果
select
*
from t12
where emp_no = 10001
and salary in (30000,4000)
order by from_date desc
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+----------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+----------------------------------+
| 1 | SIMPLE | t12 | NULL | ref | ix_t1 | ix_t1 | 5 | const | 1 | 20.00 | Using where; Backward index scan |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+----------------------------------+
可以看到 用到了 Backward index scan
如果想达到 提问者的要求 ICP 和 Backward index scan 都得用到
猜测下 Extra里的 是不是只能用一种 ？那我把desc 去掉怎样呢 ？
select
*
from t12
where emp_no = 10001
and salary in (30000,4000)
order by from_date
;
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | t12 | NULL | ref | ix_t1 | ix_t1 | 5 | const | 1 | 20.00 | Using index condition |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-----------------------+
结果果然用到了 ICP
那么现在的猜测是Extra里 不能同时用到一个以上的索引方法
既然这样，我们又有什么方法来解决这个问题 ？
这里我用到了 MySQL8.0的倒叙索引
所以我进行如下实验
create index ix_t2 on t12(emp_no,from_date desc ,salary);
select
*
from t12 force index(ix_t2)
where emp_no = 10001
and salary in (30000,4000)
order by from_date desc
;
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | t12 | NULL | ref | ix_t2 | ix_t2 | 5 | const | 1 | 20.00 | Using index condition |
+----+-------------+-------+------------+------+---------------+-------+---------+-------+------+----------+-----------------------+
结果发现既用到了ICP 也达到了倒叙的效果
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