SQL:Oracle - 笛卡尔积 - 在这种情况下

Question

阅读 Karen Morten 的 "Pro Oracle SQL"，第 9 页 "FROM clause" 说

Joins are processed in the following order

Cross Join

Inner join

Outer join

那么 Oracle 是否总是创建联接中涉及的两个 table 的笛卡尔积，无论是内部联接还是外部联接？

Table A 的值为 1 到 10（唯一），table B 的值为 (2,4,6,8)

内部联接：

select a.a1, b.b1 from a,b where a.a1 = b.b1;

外连接：

select a.a1, b.b1 from a,b where a.a1 = (+) b.b1;

根据

形成结果

创建 10*4 的笛卡尔积，然后根据条件（outer 或 equi）显示记录
不创建笛卡尔积，只创建基于连接谓词的结果集（equi join只是不确定是否可以用于外部连接，我认为笛卡尔积方法套件适用于外部连接）

Answer 1

不，它不会为内连接或外连接构建两个表的笛卡尔积。

The database uses a Cartesian join when one or more of the tables does not have any join conditions to any other tables in the statement." Database SQL Tuning Guide

因此，在您的示例中，它仅在 where 子句为空时才进行笛卡尔连接：

EXPLAIN PLAN FOR
SELECT a.a1, b.b1 FROM a,b;
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
-----------------------------------------------------------------------------
| Id  | Operation            | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |      |    40 |   240 |    10   (0)| 00:00:01 |
|   1 |  MERGE JOIN CARTESIAN|      |    40 |   240 |    10   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL  | B    |     4 |    12 |     3   (0)| 00:00:01 |
|   3 |   BUFFER SORT        |      |    10 |    30 |     7   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL | A    |    10 |    30 |     2   (0)| 00:00:01 |
-----------------------------------------------------------------------------

    SELECT a.a1, b.b1 FROM a,b;

它通常使用三种连接方法中的一种，这在相同的 document 中有解释。

在您的例子中，它使用 "hash join" 作为内部联接：

EXPLAIN PLAN FOR
SELECT a.a1, b.b1 FROM a,b WHERE a.a1 = b.b1;
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     4 |    24 |     6   (0)| 00:00:01 |
|*  1 |  HASH JOIN         |      |     4 |    24 |     6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| B    |     4 |    12 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| A    |    10 |    30 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("A"."A1"="B"."B1")

和一个 "hash join outer" 用于外部连接：

EXPLAIN PLAN FOR
SELECT a.a1, b.b1 FROM a,b WHERE a.a1 = b.b1(+);
SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY);
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |    10 |    60 |     6   (0)| 00:00:01 |
|*  1 |  HASH JOIN OUTER   |      |    10 |    60 |     6   (0)| 00:00:01 |
|   2 |   TABLE ACCESS FULL| A    |    10 |    30 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| B    |     4 |    12 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

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

   1 - access("A"."A1"="B"."B1"(+))

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