相关MYSQL DML UPDATE DELETE 中的子查询问题

2022-08-03 16:45:44

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从5.6开始MYSQL的子查询进行了大量的优化，5.5中只有EXISTS strategy，在5.7中包含如下:
IN(=ANY)
--Semi-join
--table pullout(最快的,子查询条件为唯一键)
--first match
--semi-join materialization
--loosescan
--duplicateweedout
--Materialization
--EXISTS strategy(最慢的)
NOT IN( <>ALL)
--Materialization
--EXISTS strategy(最慢的)

而(not)exist却没有任何优化还是关联子查询的方式，这和ORACLE不一样，ORACLE中in、exists
都可以使用半连接(semi)优化.所以MYSQL中尽量使用in不要用exists。not in不能使用semi-join
要小心使用，更不要用not exists，关于上面每一个含义可以参考官方手册和mariadb手册。

我们简单的看一个列子，

使用semi-join materialization优化的
mysql> explain select * from testde1 where testde1.id in(select id from testde2);
+----+--------------+-------------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table       | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra                                              |
+----+--------------+-------------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE       | | NULL       | ALL | NULL          | NULL | NULL    | NULL | NULL |   100.00 | NULL                                               |
| 1 | SIMPLE       | testde1     | NULL       | ALL | NULL          | NULL | NULL    | NULL |   15 |    10.00 | Using where; Using join buffer (Block Nested Loop) |
| 2 | MATERIALIZED | testde2     | NULL       | ALL | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL                                               |
+----+--------------+-------------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
3 rows in set, 1 warning (0.00 sec)

禁用semi join使用Materialization优化
mysql> set optimizer_switch='semijoin=off';
Query OK, 0 rows affected (0.00 sec)

mysql> explain select * from testde1 where testde1.id in(select id from testde2);
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY     | testde1 | NULL       | ALL | NULL          | NULL | NULL    | NULL |   15 |   100.00 | Using where |
| 2 | SUBQUERY    | testde2 | NULL       | ALL | NULL          | NULL | NULL    | NULL |    2 |   100.00 | NULL        |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

禁用join使用Materialization
ysql> set optimizer_switch='materialization=off';
Query OK, 0 rows affected (0.00 sec)

mysql> explain select * from testde1 where testde1.id in(select id from testde2);
+----+--------------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type        | table   | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra       |
+----+--------------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY            | testde1 | NULL       | ALL | NULL          | NULL | NULL    | NULL |   15 |   100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | testde2 | NULL       | ALL | NULL          | NULL | NULL    | NULL |    2 |    50.00 | Using where |
+----+--------------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

Note (Code 1003): /* select#1 */ select `test`.`testde1`.`id` AS `id` from `test`.`testde1` where (`test`.`testde1`.`id`,(/* select#2 */ select 1 from `test`.`testde2` where ((`test`.`testde1`.`id`) = `test`.`testde2`.`id`)))

使用DEPENDENT SUBQUERY 关联子查询优化,这也是最慢的。这和
select * from testde1 where exists (select * from testde2 where testde1.id=testde2.id);的执行计划完全一致，
testde1大表必须作为驱动表
mysql> explain select * from testde1 where exists (select * from testde2 where testde1.id=testde2.id);
+----+--------------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type        | table   | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra       |
+----+--------------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY            | testde1 | NULL       | ALL | NULL          | NULL | NULL    | NULL |   15 |   100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | testde2 | NULL       | ALL | NULL          | NULL | NULL    | NULL |    2 |    50.00 | Using where |
+----+--------------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 2 warnings (0.00 sec)

实际就是下面的执行计划：

mysql> explain delete from testde1 where id in (select id from testde2);
+----+--------------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type        | table   | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra       |
+----+--------------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | DELETE             | testde1 | NULL       | ALL | NULL          | NULL | NULL    | NULL |   15 |   100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | testde2 | NULL       | ALL | NULL          | NULL | NULL    | NULL |    2 |    50.00 | Using where |
+----+--------------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set (0.00 sec)

这里我们看到小表testde2做了驱动表。
最后来说明一下这个报错：
mysql> delete from testde1 where id in(select testde1.id from testde1,testde2 where testde1.id=testde2.id );
ERROR 1093 (HY000): You can't specify target table 'testde1' for update in FROM clause
我们先不管他有没有意义，这个报错再手册上叫做ER_UPDATE_TABLE_USED，我们首先来分析一下这个报错
这样的delete会进行exists展开那么testde1既是修改条件的来源也是修改的对象，这样是不允许的。那么如何修改呢？
实际上就需要select testde1.id from testde1,testde2 where testde1.id=testde2.id 的结果保存在一个临时表中，
不要exists展开，手册中给出的方法是
方法一、建立一个algorithm=temptable 的视图
方法二、建立一个普通视图同时修改SET optimizer_switch = 'derived_merge=off';

其目的都在于不展开选取第二种方式测试：
mysql> create view myt1
    -> as
    -> select testde1.id from testde1,testde2 where testde1.id=testde2.id;
Query OK, 0 rows affected (0.02 sec)

mysql> show status like '%tmp%';
+-------------------------+-------+
| Variable_name           | Value |
+-------------------------+-------+
| Created_tmp_disk_tables | 0     |
| Created_tmp_files       | 0     |
| Created_tmp_tables      | 2    |
+-------------------------+-------+
3 rows in set (0.01 sec)

看看执行计划：
mysql> explain delete from testde1 where id in (select * from myt1);
+----+--------------------+------------+------------+----------------+---------------+-------------+---------+------+------+----------+----------------------------------------------------+
| id | select_type        | table      | partitions | type           | possible_keys | key         | key_len | ref | rows | filtered | Extra                                              |
+----+--------------------+------------+------------+----------------+---------------+-------------+---------+------+------+----------+----------------------------------------------------+
| 1 | DELETE             | testde1    | NULL       | ALL            | NULL          | NULL        | NULL    | NULL |   13 |   100.00 | Using where                                        |
| 2 | DEPENDENT SUBQUERY | | NULL       | index_subquery |   | | 5       | func |    2 |   100.00 | Using index                                        |
| 3 | DERIVED            | testde2    | NULL       | ALL            | NULL          | NULL        | NULL    | NULL |    2 |   100.00 | NULL                                               |
| 3 | DERIVED            | testde1    | NULL       | ALL            | NULL          | NULL        | NULL    | NULL |   13 |    10.00 | Using where; Using join buffer (Block Nested Loop) |
+----+--------------------+------------+------------+----------------+---------------+-------------+---------+------+------+----------+----------------------------------------------------+
4 rows in set (0.00 sec)

先使用hash join将TESTDE2 和TESTDE1 建立为一个视图VW_NSO_1，然后使用了HASH JOIN SEMI的优化方式，明显用了到半连接优化
这也是为什么ORACLE比现在的MYSQL还是更加强劲的一个小例子，虽然都是作为一个整体，但是MYSQL已经用不到SEMI优化方式了，ORACLE
依然可以，但是可以预见不久的将来MYSQL肯定支持的。

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