MySQL 8.0 倒序索引的应用

2020-11-24 14:42  阅读 49 views 次

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
SELECTt2.column1 ,t3.column1FROM tb_test1 t1INNER JOIN tb_test2 t2 ON t2.column1 = t1.column1INNER JOIN tb_test3 ON t3.column1 = t1.column1WHERE t1.coulmn1 = #{idNo} AND t1.column2 = 'N' AND t1.column3 in ( 'A','B','C')ORDER BY t1.column4 DESCLIMIT #{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_datefrom 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 = 10001and 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 = 10001and 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 = 10001and 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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