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sql - MySQL 8 中不完整的执行计划

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我想在 MySQL 8.0 中获取查询的执行计划,但它给了我一个不完整的计划。

2018 年 9 月 8 日编辑:

通过展示一个简化示例,原始查询以未检索任何行的子查询结束。似乎 MySQL 的优化器将这些查询简化到完全修剪部分查询的程度。我修改了查询以获取有关子查询的数据。这是示例:

create table branch (
id int primary key not null,
name varchar(30) not null
);
insert into branch (id, name) values (101, 'California');
insert into branch (id, name) values (102, 'Ohio');
insert into branch (id, name) values (103, 'Delaware');

create table account (
id int primary key not null auto_increment,
balance int
);
insert into account (id, balance) values (1001, 120);
insert into account (id, balance) values (1004, 500);
insert into account (id, balance) values (1005, 45);

create table transaction (
tx_id int primary key not null auto_increment,
account_id int not null,
amount int not null,
branch_id int references branch (id)
);
insert into transaction (account_id, amount, branch_id) values
(1001, 10, 101),
(1001, 150, 101),
(1001, 200, 101),
(1001, -70, 102),
(1001, -20, 102),
(1001,-150, 102),
(1004, 50, 103),
(1004, 300, 101),
(1004, 150, 102),
(1005, 100, 102),
(1005, -55, 101);

现在的查询是:

explain
select *
from account a
join transaction t4 on t4.account_id = a.id
join branch b5 on b5.id = t4.branch_id
join (select account_id as account_id from transaction t7 where amount > 0) t6
on t6.account_id = a.id
where a.balance < 7 * (
select avg(amount) from transaction t
join branch b on b.id = t.branch_id
where t.account_id = a.id
and b.name in (select name from branch b7
where name like '%a%')
)
and a.balance < 5 * (
select max(amount)
from transaction t2
join branch b2 on b2.id = t2.branch_id
where b2.name not in (select name from branch b8
where name like '%i%')
);

现在显示(传统方案):

id select_type         table type   key     key_len ref rows filtered Extra
-- ------------------- ----- ------ ------- ------- --- ---- -------- -----
1 PRIMARY a ALL 3 33.33 Using where
1 PRIMARY t7 ALL 11 9.09 Using where
1 PRIMARY t4 ALL 11 10 Using where
1 PRIMARY b5 eq_ref PRIMARY 4 ... 1 100
5 SUBQUERY b2 ALL 3 100 Using where
5 SUBQUERY t2 ALL 11 10 Using where
6 DEPENDENT SUBQUERY b8 ALL 3 33.33 Using where
3 DEPENDENT SUBQUERY b7 ALL 3 33.33 Using where
3 DEPENDENT SUBQUERY t ALL 11 10 Using where
3 DEPENDENT SUBQUERY b eq_ref PRIMARY 4 ... 1 33.33 Using where

它现在显示除标量子查询 t6 之外的所有表的信息。它在哪里?

最佳答案

我尝试测试您的查询,但我在任何表中都有零行。 EXPLAIN 显示“Impossible WHERE noticeed after reading const tables”,这意味着没有满足查询条件的行。

在我的测试中,我看到了 t2、b2、b8、t、b、b7、t7,但没有看到 a、t4、b5、t6。如果不读取表,它似乎会从 EXPLAIN 中省略表,因为查询条件意味着读取它们没有意义,因为它们保证不匹配任何行。

我看不出这个条款有任何合乎逻辑的目的:

join (select max(account_id) as account_id from transaction t7) t6 
on t6.account_id = a.id

如果我从查询中取出这个连接,我会得到一个没有“Impossible WHERE”注释的 EXPLAIN,并且它具有所有其他相关名称:

+----+--------------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------------------------+
| 1 | PRIMARY | a | ALL | PRIMARY | NULL | NULL | NULL | 1 | Using where |
| 1 | PRIMARY | t4 | ALL | NULL | NULL | NULL | NULL | 2 | Using where; Using join buffer (Block Nested Loop) |
| 1 | PRIMARY | b5 | ALL | PRIMARY | NULL | NULL | NULL | 2 | Using where; Using join buffer (Block Nested Loop) |
| 4 | SUBQUERY | t2 | ALL | NULL | NULL | NULL | NULL | 2 | NULL |
| 4 | SUBQUERY | b2 | ALL | PRIMARY | NULL | NULL | NULL | 2 | Using where; Using join buffer (Block Nested Loop) |
| 5 | DEPENDENT SUBQUERY | b8 | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
| 2 | DEPENDENT SUBQUERY | t | ALL | NULL | NULL | NULL | NULL | 2 | Using where |
| 2 | DEPENDENT SUBQUERY | b | ALL | PRIMARY | NULL | NULL | NULL | 2 | Using where; Using join buffer (Block Nested Loop) |
| 2 | DEPENDENT SUBQUERY | b7 | ALL | NULL | NULL | NULL | NULL | 2 | Using where; FirstMatch(b); Using join buffer (Block Nested Loop) |
+----+--------------------+-------+------+---------------+------+---------+------+------+-------------------------------------------------------------------+

我没有像我在你的表中猜测的那样创建任何索引,所以这个 EXPLAIN 没有显示优化。但至少所有相关名称都出现了。

关于sql - MySQL 8 中不完整的执行计划,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52119462/

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