ToprowDB Dynamic Server 查询优化技术---子查询优化--02-4
1.2.2 PostgreSQL
1.2.2.1 S6语句
查看查询执行计划，子查询被优化（采用物化的方式，使得子查询结果被缓存，既子查询只执行一次）。
postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 >= ANY (SELECT b1 FROM t1);
Nested Loop Semi Join (cost=0.00..41707.39 rows=680 width=12)
Join Filter: (t3.b3 >= t1.b1)
-> Seq Scan on t3 (cost=0.00..30.40 rows=2040 width=12)
-> Materialize (cost=0.00..40.60 rows=2040 width=4)
-> Seq Scan on t1 (cost=0.00..30.40 rows=2040 width=4)
1.2.2.2 S7语句
查看查询执行计划，子查询被优化（采用半连接）。
postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 >= ANY (SELECT a1 FROM t1);
Nested Loop Semi Join (cost=0.15..377.82 rows=680 width=12)
-> Seq Scan on t3 (cost=0.00..30.40 rows=2040 width=12)
-> Index Only Scan using t1_a1_key on t1 (cost=0.15..12.08 rows=680 width=4)
Index Cond: (a1 <= t3.b3)
1.2.2.3 S8语句
查看查询执行计划，子查询被优化（采用半连接）。
postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 <= SOME (SELECT a1 FROM t1);
Nested Loop Semi Join (cost=0.15..377.82 rows=680 width=12)
-> Seq Scan on t3 (cost=0.00..30.40 rows=2040 width=12)
-> Index Only Scan using t1_a1_key on t1 (cost=0.15..12.08 rows=680 width=4)
Index Cond: (a1 >= t3.b3)
1.2.2.4 S9语句
查看查询执行计划，子查询被优化。
postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 = SOME (SELECT a1 FROM t1);
Hash Semi Join (cost=55.90..103.00 rows=1020 width=12)
Hash Cond: (t3.b3 = t1.a1)
-> Seq Scan on t3 (cost=0.00..30.40 rows=2040 width=12)
-> Hash (cost=30.40..30.40 rows=2040 width=4)
-> Seq Scan on t1 (cost=0.00..30.40 rows=2040 width=4)
1.2.2.5 S10语句
查看查询执行计划，子查询没有被优化。
postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 <= ALL (SELECT a1 FROM t1);
Seq Scan on t3 (cost=0.00..46649.50 rows=1020 width=12)
Filter: (SubPlan 1)
SubPlan 1
-> Materialize (cost=0.00..40.60 rows=2040 width=4)
-> Seq Scan on t1 (cost=0.00..30.40 rows=2040 width=4)
1.2.2.6 S11语句
查看查询执行计划，子查询被优化（采用半连接）。
查看查询执行计划，子查询没有被优化。
postgres=# EXPLAIN SELECT * FROM t3 WHERE b3 <= ALL (SELECT a1 FROM t1 WHERE a3=a1);
Seq Scan on t3 (cost=0.00..8527.00 rows=1020 width=12)
Filter: (SubPlan 1)
SubPlan 1
-> Index Only Scan using t1_a1_key on t1 (cost=0.15..8.17 rows=1 width=4)
Index Cond: (a1 = t3.a3)
1.2.3 MySQL
1.2.3.1 S6语句
查看查询执行计划，子查询没有被优化。
mysql> EXPLAIN SELECT * FROM t3 WHERE b3 >= ANY (SELECT b1 FROM t1);
+----+-------------+-------+------+------+-------------+
| id | select_type | table | type | key | Extra |
+----+-------------+-------+------+------+-------------+
| 1 | PRIMARY | t3 | ALL | NULL | Using where |
| 2 | SUBQUERY | t1 | ALL | NULL | NULL |
+----+-------------+-------+------+------+-------------+
2 rows in set (0.04 sec)
1.2.3.2 S7语句
查看查询执行计划，子查询没有被优化（但注意“Select tables optimized away”，表明子查询被使用特有的技术进行了优化，但不是常规的子查询被上拉的优化方式）。
mysql> EXPLAIN SELECT * FROM t3 WHERE b3 >= ANY (SELECT a1 FROM t1);
+----+-------------+-------+------+------+------------------------------+
| id | select_type | table | type | key | Extra |
+----+-------------+-------+------+------+------------------------------+
| 1 | PRIMARY | t3 | ALL | NULL | Using where |
| 2 | SUBQUERY | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+------+------------------------------+
2 rows in set (0.00 sec)
1.2.3.3 S8语句
查看查询执行计划，子查询被优化（采用半连接）。
mysql> EXPLAIN SELECT * FROM t3 WHERE b3 <= SOME (SELECT a1 FROM t1);
+----+-------------+-------+------+------+------------------------------+
| id | select_type | table | type | key | Extra |
+----+-------------+-------+------+------+------------------------------+
| 1 | PRIMARY | t3 | ALL | NULL | Using where |
| 2 | SUBQUERY | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+-----+------------------------------+
2 rows in set (0.00 sec)
1.2.3.4 S9语句
查看查询执行计划，子查询被优化。
mysql> EXPLAIN SELECT * FROM t3 WHERE b3 = SOME (SELECT a1 FROM t1);
+----+-------------+-------+-------+------+----------------------------------------------------+
| id | select_type | table | type | key | Extra |
+----+-------------+-------+-------+------+----------------------------------------------------+
| 1 | SIMPLE | t1 | index | a1 | Using index |
| 1 | SIMPLE | t3 | ALL | NULL | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+------+----------------------------------------------------+
2 rows in set (0.00 sec)
1.2.3.5 S10语句
查看查询执行计划，子查询没有被优化。
mysql> EXPLAIN SELECT * FROM t3 WHERE b3 <= ALL (SELECT a1 FROM t1);
+----+-------------+-------+-------+------+-------------+
| id | select_type | table | type | key | Extra |
+----+-------------+-------+-------+------+-------------+
| 1 | PRIMARY | t3 | ALL | NULL | Using where |
| 2 | SUBQUERY | t1 | index | a1 | Using index |
+----+-------------+-------+-------+------+-------------+
2 rows in set (0.00 sec)
1.2.3.6 S11语句
查看查询执行计划，子查询没有被优化。
mysql> EXPLAIN SELECT * FROM t3 WHERE b3 <= ALL (SELECT a1 FROM t1 WHERE a3=a1);
+----+--------------------+-------+------+------+--------------------------+
| id | select_type | table | type | key | Extra |
+----+--------------------+-------+------+------+--------------------------+
| 1 | PRIMARY | t3 | ALL | NULL | Using where |
| 2 | DEPENDENT SUBQUERY | t1 | ref | a1 | Using where; Using index |
+----+--------------------+-------+------+------+--------------------------+
2 rows in set (0.00 sec)
1.2.4 对比
我们把原始的SQL语句复制一份，目的是查阅方便。
S6: SELECT * FROM t3 WHERE b3 >= ANY (SELECT b1 FROM t1);
S7: SELECT * FROM t3 WHERE b3 >= ANY (SELECT a1 FROM t1);
S8: SELECT * FROM t3 WHERE b3 <= SOME (SELECT a1 FROM t1);
S9: SELECT * FROM t3 WHERE b3 = SOME (SELECT a1 FROM t1);
S10: SELECT * FROM t3 WHERE b3 <= ALL (SELECT a1 FROM t1);
S11: SELECT * FROM t3 WHERE b3 <= ALL (SELECT a1 FROM t1 WHERE a3=a1);
然后对比如下:
SQL语句
语句特点
ToprowDB
PostgreSQL
MySQL
S6
非相关子查询，>= ANY操作，b1普通列
Join优化
Semi Join优化+ 物化（非相关子查询）
没有被优化
S7
非相关子查询，>= ANY操作，a1索引键
Join优化
Semi Join优化+ 索引扫描
特定的优化
S8
非相关子查询，<= SOME操作，a1索引键
Join优化
Semi Join优化+ 索引扫描
特定的优化
S9
非相关子查询，= SOME操作，a1索引键
没有被优化
Semi Join优化
被优化
S10
非相关子查询，<= ALL操作，a1索引键
没有被优化
没有被优化
特定的优化
S11
相关子查询，<= ALL操作，a1索引键，条件a3=a1为索引键
Semi Join优化
没有被优化
特定的优化
分析：
q 从整体上看，对于ANY/SOME/ALL类型的子查询，PostgreSQL、MySQL、ToprowDB各有特色，MySQL表面上没有上拉子查询显得弱一些，但是内部启用了索引在子查询的目标列上进行聚集运算（利用索引求大值小值，对比S6和S7），极大地加快了执行速度，这也是上面的表中用“特定的优化”表达的含义
q 对比S8和S9，可以看出，ToprowDB对S9没有提供优化，而PostgreSQL、MySQL能够优化，这一点ToprowDB需要努力
q 对比S11，ToprowDB和MySQL强于PostgreSQL
在子查询优化的篇中，我们发现MySQL的IN子查询优化技术高于其他数据库，而本篇重，EXISTS类型的子查询，MySQL又弱于PostgreSQL、ToprowDB ，而ANY/SOME/ALL类型的子查询三者几乎打平，所以我们可以认为三者子查询的优化能力各有所长。
1.3 还有其他类型的子查询吗？
前面几节，看到了形形色色的各种子查询语句，种类众多显得琳琅满目，子查询的内容和形式十分地丰富多彩。但是，这些尚没有囊括到所有的类型，比如NOT IN、NOT EXISTS、各种ANY（=ANY、>ANY、>=ANY、<ANY、<=ANY）和类型ANY的各种SOME、各种ALL等等。
这些格式的子查询，我们不再举例，留待大家自己探讨。
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版权声明：本文为CSDN博主「那海蓝蓝」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。
原文链接：https://blog.csdn.net/fly2nn/article/details/61924795
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