一个RDBMS左连接SQL执行计划解析

1、测试数据如下：

SQL> select * from t1;
  a | b  | c

---+----+---
  1 | 10 | 1
  2 | 20 | 2
  3 | 30 | 3
  4 | 40 | 4
  5 | 50 | 5
  6 | 60 | 6

(6 rows)

SQL> select * from t2;
  a | b  | d

---+----+---
  1 | 10 | 1
  2 | 20 | 2
  3 | 30 | 3

(3 rows)

2、解析示例SQL 如下 ：

select *

from (

select * from t1 where c >= 2

) t1 left join (

select * from t2 where b < 30

) t2 on t1.a = t2.a

and t2.d > 1

where t1.b < 50

;

3、Oracle数据库查看执行结果及执行计划：

SQL> select *

from (

select * from t1 where c >= 2

) t1 left join (

select * from t2 where b < 30

) t2 on t1.a = t2.a

and t2.d > 1

where t1.b < 50

;

A        B           C      A         B        D

---------- ---------- ---------- ---------- ---------- ----------
      2       20           2      2        20        2
      3       30           3
      4       40           4

Execution Plan

----------------------------------------------------------

Plan hash value: 1823443478

---------------------------------------------------------------------------

| Id  | Operation       | Name | Rows  | Bytes | Cost (%CPU)| Time      |

---------------------------------------------------------------------------

|   0 | SELECT STATEMENT   |      |    3 |   234 |    7  (15)| 00:00:01 |

|*  1 |  HASH JOIN OUTER   |      |    3 |   234 |    7  (15)| 00:00:01 |

|*  2 |   TABLE ACCESS FULL| T1   |    3 |   117 |    3   (0)| 00:00:01 |

|*  3 |   TABLE ACCESS FULL| T2   |    1 |    39 |    3   (0)| 00:00:01 |

---------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

1 - access("T1"."A"="T2"."A"(+))
    2 - filter("T1"."B"<50 AND "C">=2)
    3 - filter("T2"."D"(+)>1 AND "B"(+)<30)

Note

-----
    - dynamic sampling used for this statement (level=2)

Statistics

----------------------------------------------------------
       0  recursive calls
       0  db block gets
       7  consistent gets
       0  physical reads
       0  redo size
     926  bytes sent via SQL*Net to client
     523  bytes received via SQL*Net from client
       2  SQL*Net roundtrips to/from client
       0  sorts (memory)
       0  sorts (disk)
       3  rows processed

4、PGSQL数据库查看执行结果及执行计划：
      

postgres=# select *

postgres-# from (

postgres(# select * from t1 where c >= 2

postgres(# ) t1 left join (

postgres(# select * from t2 where b < 30

postgres(# ) t2 on t1.a = t2.a

postgres-# and t2.d > 1

postgres-# where t1.b < 50

postgres-# ;
  a | b  | c | a | b  | d

---+----+---+---+----+---
  2 | 20 | 2 | 2 | 20 | 2
  3 | 30 | 3 |   |    | 
  4 | 40 | 4 |   |    | 

(3 rows)

postgres=# explain analyze select *

postgres-# from (

postgres(# select * from t1 where c >= 2

postgres(# ) t1 left join (

postgres(# select * from t2 where b < 30

postgres(# ) t2 on t1.a = t2.a

postgres-# and t2.d > 1

postgres-# where t1.b < 50

postgres-# ;
                                                  QUERY PLAN                                                

------------------------------------------------------------------------------------------------------------
  Hash Left Join  (cost=37.04..85.88 rows=197 width=24) (actual time=0.020..0.027 rows=3 loops=1)
    Hash Cond: ("outer".a = "inner".a)
    ->  Seq Scan on t1  (cost=0.00..36.55 rows=197 width=12) (actual time=0.005..0.008 rows=3 loops=1)
          Filter: ((c >= 2) AND (b < 50))
    ->  Hash  (cost=36.55..36.55 rows=197 width=12) (actual time=0.006..0.006 rows=1 loops=1)
          ->  Seq Scan on t2  (cost=0.00..36.55 rows=197 width=12) (actual time=0.002..0.003 rows=1 loops=1)
                Filter: ((b < 30) AND (d > 1))
  Total runtime: 0.052 ms

(8 rows)

5、MySQL数据库查看执行结果及执行计划：
      

mysql> select *
     -> from (
     -> select * from t1 where c >= 2
     -> ) t1 left join (
     -> select * from t2 where b < 30
     -> ) t2 on t1.a = t2.a
     -> and t2.d > 1
     -> where t1.b < 50
     -> ;

+---+----+---+------+------+------+

| a | b  | c | a    | b    | d    |

+---+----+---+------+------+------+

| 2 | 20 | 2 |    2 |   20 |    2 |

| 3 | 30 | 3 | NULL | NULL | NULL |

| 4 | 40 | 4 | NULL | NULL | NULL |

+---+----+---+------+------+------+

3 rows in set (0.05 sec)

mysql> explain select *
     -> from (
     -> select * from t1 where c >= 2
     -> ) t1 left join (
     -> select * from t2 where b < 30
     -> ) t2 on t1.a = t2.a
     -> and t2.d > 1
     -> where t1.b < 50
     -> ;

+----+-------------+------------+------+---------------+-------------+---------+------+------+-------------+

| id | select_type | table      | type | possible_keys | key         | key_len | ref  | rows | Extra       |

+----+-------------+------------+------+---------------+-------------+---------+------+------+-------------+

|  1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL        | NULL    | NULL |    6 | Using where |

|  1 | PRIMARY     | <derived3> | ref  | <auto_key0>   | <auto_key0> | 8       | t1.a |    1 | Using where |

|  3 | DERIVED     | t2         | ALL  | NULL          | NULL        | NULL    | NULL |    3 | Using where |

|  2 | DERIVED     | t1         | ALL  | NULL          | NULL        | NULL    | NULL |    6 | Using where |

+----+-------------+------------+------+---------------+-------------+---------+------+------+-------------+

4 rows in set (0.00 sec)

6、针对以上SQL执行计划的分析：

1) 全表扫描左表T1，同时根据T1表子查询条件"C">=2和where过滤条件"T1"."B"<50联合过滤，即filter("T1"."B"<50 AND "C">=2)，计算结果临时表记为tmp1；

2) 全表扫描右表T2，同时根据T2表子查询条件"B"(+)<30和on子句"T2"."D"(+)>1联合过滤，即filter("T2"."D"(+)>1 AND "B"(+)<30)，计算结果临时表记为tmp2；

3) 左表T1及右表T2处理后临时表tmp1和tmp2通过access("T1"."A"="T2"."A"(+))连接条件进行Hash Left Join操作，左临时表结果集全量返回，右表不匹配行置为null，返回结果临时表记为tmp3；

4) 返回结果集。

7、一些更为复杂得SQL如下，有兴趣自行研究：

1) 测试数据

create table tmp1 as

select a,b,c,a as e from t1;

create table tmp2 as

select a,b,d,a as e from t2;

2) 示例SQL

select *

from (

select * from tmp1 where c >= 1

) t1 left join (

select * from tmp2 where b < 30

) t2 on t1.a = t2.a

and t2.d > 1 and t1.e >= 2

where t1.b < 50

;

select *

from (

select * from tmp1 where c >= 1

) t1 left join (

select * from tmp2 where b < 30

) t2 on t1.a = t2.a

and t2.d > 1 and t1.e >= 2

where t1.b < 50 and t2.e <= 3

;