我发现我认为是 Oracle 中的一个错误，但我想知道是否有我遗漏的文档记录。

fiddle : 甲骨文:http://sqlfiddle.com/#!4/43c19/2 SQL Server:http://sqlfiddle.com/#!3/ddc49/1 MySql:http://sqlfiddle.com/#!2/43c195/1

基本上我有一个主表，我将其加入到辅助表中。然后我离开了一个 View 。如果我在 View 的连接中指定我只想在辅助表中的列不为空时加入，我会得到意想不到的结果。最好通过显示查询来解释这一点:

SELECT
  1,
  MainTable.*
FROM
  MainTable
  LEFT JOIN SecondaryTable ON MainTable.KeyColumn = SecondaryTable.KeyColumn
  LEFT JOIN ViewWithoutSecondary ON ((SecondaryTable.KeyColumn IS NOT NULL) AND SecondaryTable.KeyColumn = ViewWithoutSecondary.KeyColumn)
UNION ALL
SELECT
  2,
  MainTable.*
FROM
  MainTable
  LEFT JOIN SecondaryTable ON MainTable.KeyColumn = SecondaryTable.KeyColumn
  LEFT JOIN ViewWithSecondary ON ((SecondaryTable.KeyColumn IS NOT NULL) AND SecondaryTable.KeyColumn = ViewWithSecondary.KeyColumn)

请参阅下面的创建脚本以自行测试。在 SQL Server 和 MySql 中，我得到相同的结果，但 Oracle 不同。架构中有三个表和两个 View 。 View 定义如下:

CREATE VIEW ViewWithoutSecondary
AS
SELECT
  TertiaryTable.KeyColumn,
  TertiaryValue + 1 ViewValue
FROM
  TertiaryTable

CREATE VIEW ViewWithSecondary
AS
SELECT
  SecondaryTable.KeyColumn,
  TertiaryValue + 1 ViewValue
FROM
  SecondaryTable
  LEFT JOIN TertiaryTable ON SecondaryTable.KeyColumn = TertiaryTable.KeyColumn;

在 Oracle 中，我发现如果 View 包含对 SecondaryTable 的引用，那么我只会从 MainTable 中获取与 Secondary 表匹配的行。在我看来，Oracle 正在以某种方式内联 View 代码，以便省略其中一行。

我认为，如果 MainTable 有三行，那么对其进行两次左连接应该总是至少返回三行，加上连接的任何结果。然而，在给出的示例中，情况并非如此。

我知道 SecondaryTable.KeyValue IS NOT NULL 是多余的，因为如果值为空，子句的后半部分将不正确，但我一直在尝试重新编写查询以帮助优化器提出更好的计划。

运行示例的完整创建脚本是:

CREATE TABLE MainTable
(
  KeyColumn varchar(32),
  ValueColumn varchar(32)
);

INSERT INTO MainTable VALUES ('123', 'abc');
INSERT INTO MainTable VALUES ('456', 'def');
INSERT INTO MainTable VALUES ('789', 'ghi');

CREATE TABLE SecondaryTable
(
  KeyColumn varchar(32),
  SecondaryValue integer  
);

INSERT INTO SecondaryTable VALUES ('123', 1);
INSERT INTO SecondaryTable VALUES ('456', 2);

CREATE TABLE TertiaryTable
(
  KeyColumn varchar(32),
  TertiaryValue integer  
);

INSERT INTO TertiaryTable VALUES ('123', 1);

CREATE VIEW ViewWithoutSecondary
AS
SELECT
  TertiaryTable.KeyColumn,
  TertiaryValue + 1 ViewValue
FROM
  TertiaryTable;

CREATE VIEW ViewWithSecondary
AS
SELECT
  SecondaryTable.KeyColumn,
  TertiaryValue + 1 ViewValue
FROM
  SecondaryTable
  LEFT JOIN TertiaryTable ON SecondaryTable.KeyColumn = TertiaryTable.KeyColumn;

最佳答案

如果您对查询运行解释计划，您可以看到 Oracle 正在通过内联 View 来转换查询，并且出于某种原因，它在第 2 行执行内部联接，而不是左外联接。

explain plan 
SET statement_id = 'no-hint' FOR  
SELECT 
  MainTable.*
FROM
  MainTable
  LEFT JOIN SecondaryTable ON MainTable.KeyColumn = SecondaryTable.KeyColumn
  LEFT JOIN ViewWithSecondary ON ((SecondaryTable.KeyColumn IS NOT NULL) AND SecondaryTable.KeyColumn = ViewWithSecondary.KeyColumn);

SELECT PLAN_TABLE_OUTPUT 
  FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'no-hint','TYPICAL'));


----------------------------------------------------------------------------------------
| Id  | Operation             | Name           | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                |     2 |   108 |    20  (10)| 00:00:01 |
|   1 |  NESTED LOOPS OUTER   |                |     2 |   108 |    20  (10)| 00:00:01 |
|*  2 |   HASH JOIN           |                |     2 |   108 |     7  (15)| 00:00:01 |
|   3 |    TABLE ACCESS FULL  | SECONDARYTABLE |     2 |    36 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL  | MAINTABLE      |     3 |   108 |     3   (0)| 00:00:01 |
|   5 |   VIEW                |                |     1 |       |     7  (15)| 00:00:01 |
|*  6 |    FILTER             |                |       |       |            |          |
|*  7 |     HASH JOIN OUTER   |                |     1 |    36 |     7  (15)| 00:00:01 |
|*  8 |      TABLE ACCESS FULL| SECONDARYTABLE |     1 |    18 |     3   (0)| 00:00:01 |
|   9 |      TABLE ACCESS FULL| TERTIARYTABLE  |     1 |    18 |     3   (0)| 00:00:01 |
----------------------------------------------------------------------------------------

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

   2 - access("MAINTABLE"."KEYCOLUMN"="SECONDARYTABLE"."KEYCOLUMN")
   6 - filter("SECONDARYTABLE"."KEYCOLUMN" IS NOT NULL)
   7 - access("SECONDARYTABLE"."KEYCOLUMN"="TERTIARYTABLE"."KEYCOLUMN"(+))
   8 - filter("SECONDARYTABLE"."KEYCOLUMN"="SECONDARYTABLE"."KEYCOLUMN")

解决此问题的方法是使用 NO_MERGE 提示。

SELECT /*+ NO_MERGE(ViewWithSecondary) */
  MainTable.*
FROM
  MainTable
  LEFT JOIN SecondaryTable ON MainTable.KeyColumn = SecondaryTable.KeyColumn
  LEFT JOIN ViewWithSecondary ON ((SecondaryTable.KeyColumn IS NOT NULL) AND SecondaryTable.KeyColumn = ViewWithSecondary.KeyColumn);

这会产生预期的结果:

KEYCOLUMN                        VALUECOLUMN                    
-------------------------------- --------------------------------
123                              abc                              
456                              def                              
789                              ghi  

比较提示查询的查询计划。在这里，我们在第 2 行看到一个左外连接。

explain plan
SET statement_id = 'with-hint' FOR
SELECT /*+ NO_MERGE(ViewWithSecondary) */
  MainTable.*
FROM
  MainTable
  LEFT JOIN SecondaryTable ON MainTable.KeyColumn = SecondaryTable.KeyColumn
  LEFT JOIN ViewWithSecondary ON ((SecondaryTable.KeyColumn IS NOT NULL) AND SecondaryTable.KeyColumn = ViewWithSecondary.KeyColumn);

SELECT PLAN_TABLE_OUTPUT 
  FROM TABLE(DBMS_XPLAN.DISPLAY(NULL, 'with-hint','TYPICAL'));  

--------------------------------------------------------------------------------------------
| Id  | Operation              | Name              | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |                   |     6 |   324 |    26   (8)| 00:00:01 |
|   1 |  NESTED LOOPS OUTER    |                   |     6 |   324 |    26   (8)| 00:00:01 |
|*  2 |   HASH JOIN OUTER      |                   |     3 |   162 |     7  (15)| 00:00:01 |
|   3 |    TABLE ACCESS FULL   | MAINTABLE         |     3 |   108 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL   | SECONDARYTABLE    |     2 |    36 |     3   (0)| 00:00:01 |
|   5 |   VIEW                 |                   |     2 |       |     7  (15)| 00:00:01 |
|*  6 |    FILTER              |                   |       |       |            |          |
|*  7 |     VIEW               | VIEWWITHSECONDARY |     2 |    36 |     7  (15)| 00:00:01 |
|*  8 |      HASH JOIN OUTER   |                   |     2 |    72 |     7  (15)| 00:00:01 |
|   9 |       TABLE ACCESS FULL| SECONDARYTABLE    |     2 |    36 |     3   (0)| 00:00:01 |
|  10 |       TABLE ACCESS FULL| TERTIARYTABLE     |     1 |    18 |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------

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

   2 - access("MAINTABLE"."KEYCOLUMN"="SECONDARYTABLE"."KEYCOLUMN"(+))
   6 - filter("SECONDARYTABLE"."KEYCOLUMN" IS NOT NULL)
   7 - filter("SECONDARYTABLE"."KEYCOLUMN"="VIEWWITHSECONDARY"."KEYCOLUMN")
   8 - access("SECONDARYTABLE"."KEYCOLUMN"="TERTIARYTABLE"."KEYCOLUMN"(+))

关于mysql - 将表连接到依赖于表的 View 时，这是 Oracle 中的错误吗，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/27903303/