两个左连接SQL执行计划解析（Oracle和PGSQL对比）：

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wcwen1990 2021-08-06 16:29:19 博主文章分类：MySQL ©著作权

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上一篇解析链接如下：
https://www.cnblogs.com/wcwen1990/p/9325968.html

1、SQL示例1：

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
;

     A        B           C      E         A        B       D          E
---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
      2       20           2      2         2           20       2          2
      4       40           4      4
      3       30           3      3
      1       10           1      1

Execution Plan
----------------------------------------------------------
Plan hash value: 2592321047

---------------------------------------------------------------------------
| Id | Operation       | Name | Rows | Bytes | Cost (%CPU)| Time      |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |    4 |   416 |    7 (15)| 00:00:01 |
|* 1 | HASH JOIN OUTER   |      |    4 |   416 |    7 (15)| 00:00:01 |
|* 2 |   TABLE ACCESS FULL| TMP1 |    4 |   208 |    3   (0)| 00:00:01 |
|* 3 |   TABLE ACCESS FULL| TMP2 |    1 |    52 |    3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("TMP1"."A"="TMP2"."A"(+))
        filter("TMP1"."E">=CASE    WHEN ("TMP2"."A"(+) IS NOT NULL) THEN 2
           ELSE 2 END )
    2 - filter("TMP1"."B"<50 AND "C">=1)
    3 - filter("TMP2"."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
        1082 bytes sent via SQL*Net to client
     524 bytes received via SQL*Net from client
       2 SQL*Net roundtrips to/from client
       0 sorts (memory)
       0 sorts (disk)
       4 rows processed

postgres=# explain analyze select *
postgres-# from (
postgres(# select * from tmp1 where c >= 1
postgres(# ) t1 left join (
postgres(# select * from tmp2 where b < 30
postgres(# ) t2 on t1.a = t2.a
postgres-# and t2.d > 1 and t1.e >= 2
postgres-# where t1.b < 50
postgres-# ;
                                                   QUERY PLAN
--------------------------------------------------------------------------------------------------------------
Hash Left Join (cost=34.90..80.00 rows=181 width=32) (actual time=0.021..0.035 rows=4 loops=1)
    Hash Cond: ("outer".a = "inner".a)
    Join Filter: ("outer".e >= 2)
    -> Seq Scan on tmp1 (cost=0.00..34.45 rows=181 width=16) (actual time=0.006..0.011 rows=4 loops=1)
          Filter: ((c >= 1) AND (b < 50))
    -> Hash (cost=34.45..34.45 rows=181 width=16) (actual time=0.007..0.007 rows=1 loops=1)
          -> Seq Scan on tmp2 (cost=0.00..34.45 rows=181 width=16) (actual time=0.002..0.003 rows=1 loops=1)
                Filter: ((b < 30) AND (d > 1))
Total runtime: 0.063 ms
(9 rows)

SQL执行计划的分析：

1) 全表扫描左表TMP1，同时根据TMP1表子查询条件"C">=1和where过滤条件"T1"."B"<50联合过滤，即filter("TMP1"."B"<50 AND "C">=1)，计算结果临时表记为tmp1；
2) 全表扫描右表TMP2，同时根据TMP2表子查询条件"B"(+)<30和on子句"T2"."D"(+)>1联合过滤，即filter("TMP2"."D"(+)>1 AND "B"(+)<30)，计算结果临时表记为tmp2；
3) 左表TMP1及右表TMP2处理后临时表tmp1和tmp2通过access("TMP1"."A"="TMP2"."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。

2、SQL示例2：

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 and t2.e <= 3
;

A B C E A B D E
---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
2 20 2 2 2 20 2 2

Execution Plan
----------------------------------------------------------
Plan hash value: 1630095649

---------------------------------------------------------------------------
| Id | Operation       | Name | Rows | Bytes | Cost (%CPU)| Time      |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |    1 |   104 |    7 (15)| 00:00:01 |
|* 1 | HASH JOIN       |      |    1 |   104 |    7 (15)| 00:00:01 |
|* 2 |   TABLE ACCESS FULL| TMP2 |    1 |    52 |    3   (0)| 00:00:01 |
|* 3 |   TABLE ACCESS FULL| TMP1 |    3 |   156 |    3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("TMP1"."A"="TMP2"."A")
    2 - filter("TMP2"."E"<=3 AND "TMP2"."D">1 AND "B"<30)
    3 - filter("TMP1"."B"<50 AND "TMP1"."E">=2 AND "C">=1)

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

Statistics
----------------------------------------------------------
       9 recursive calls
       0 db block gets
      15 consistent gets
       0 physical reads
       0 redo size
     981 bytes sent via SQL*Net to client
     524 bytes received via SQL*Net from client
       2 SQL*Net roundtrips to/from client
       0 sorts (memory)
       0 sorts (disk)
       1 rows processed

SQL>

postgres=# select *
postgres-# from (
postgres(# select * from tmp1 where c >= 1
postgres(# ) t1 left join (
postgres(# select * from tmp2 where b < 30
postgres(# ) t2 on t1.a = t2.a
postgres-# and t2.d > 1 and t1.e >= 2
postgres-# where t1.b < 50 and t2.e <= 3
postgres-# ;
a | b | c | e | a | b | d | e
---+----+---+---+---+----+---+---
2 | 20 | 2 | 2 | 2 | 20 | 2 | 2
(1 row)

postgres=# explain analyze select *
postgres-# from (
postgres(# select * from tmp1 where c >= 1
postgres(# ) t1 left join (
postgres(# select * from tmp2 where b < 30
postgres(# ) t2 on t1.a = t2.a
postgres-# and t2.d > 1 and t1.e >= 2
postgres-# where t1.b < 50 and t2.e <= 3
postgres-# ;
                                                  QUERY PLAN
-------------------------------------------------------------------------------------------------------------
Hash Join (cost=38.68..78.43 rows=18 width=32) (actual time=0.033..0.041 rows=1 loops=1)
    Hash Cond: ("outer".a = "inner".a)
    -> Seq Scan on tmp1 (cost=0.00..38.53 rows=60 width=16) (actual time=0.007..0.011 rows=3 loops=1)
          Filter: ((c >= 1) AND (e >= 2) AND (b < 50))
    -> Hash (cost=38.53..38.53 rows=60 width=16) (actual time=0.008..0.008 rows=1 loops=1)
          -> Seq Scan on tmp2 (cost=0.00..38.53 rows=60 width=16) (actual time=0.003..0.005 rows=1 loops=1)
                Filter: ((b < 30) AND (d > 1) AND (e <= 3))
Total runtime: 0.070 ms
(8 rows)

postgres=#

SQL执行计划的分析：

1) 全表扫描左表TMP2，同时根据TMP2表子查询条件"B"<30和where过滤条件"TMP2"."E"<=3及ON子句过滤条件"TMP2"."D">1联合过滤，即filter("TMP2"."E"<=3 AND "TMP2"."D">1 AND "B"<30)，计算结果临时表记为tmp1；
2) 全表扫描右表TMP1，同时根据TMP1表子查询条件"C">=1和where子句过滤条件"TMP1"."B"<50及ON子句"TMP1"."E">=2联合过滤，即filter("TMP1"."B"<50 AND "TMP1"."E">=2 AND "C">=1)，计算结果临时表记为tmp2；
3) 临时表tmp1和tmp2通过access("TMP1"."A"="TMP2"."A")连接条件进行Hash Join连接操作（此处left join写法已经被转换为内链接），返回匹配结果临时表记为tmp3；
4) 返回结果集。