自然连接（NATURAL JOIN）是一种特殊的等价连接，它将表中具有相同名称的列自动进行记录匹配。自然连接不必指定任何同等连接条件。这篇文章讲的一个关于natural join 的bug！（由 dingjun123 提示！）
SQL> conn store/yang
已连接。
SQL> create table a as select * from all_objects;
表已创建。
SQL> set timing on
SQL> create table b as select * from all_objects;
表已创建。
已用时间: 00: 00: 20.36
SQL> set autot on
SQL> set linesize 100
SQL> select count(*) from a natural join b;
COUNT(*)
----------
0 ---错误的结果！
已用时间: 00: 00: 00.04
执行计划
----------------------------------------------------------
Plan hash value: 1397777030
------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 316 | | 1500 (1)| 00:00:18 |
| 1 | SORT AGGREGATE | | 1 | 316 | | | |
|* 2 | HASH JOIN | | 1 | 316 | 8184K| 1500 (1)| 00:00:18 |
| 3 | TABLE ACCESS FULL| A | 49280 | 7603K| | 281 (1)| 00:00:04 |
| 4 | TABLE ACCESS FULL| B | 66983 | 10M| | 282 (1)| 00:00:04 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("A"."EDITION_NAME"="B"."EDITION_NAME" AND
"A"."NAMESPACE"="B"."NAMESPACE" AND "A"."SECONDARY"="B"."SECONDARY" AND
"A"."GENERATED"="B"."GENERATED" AND "A"."TEMPORARY"="B"."TEMPORARY" AND
"A"."STATUS"="B"."STATUS" AND "A"."TIMESTAMP"="B"."TIMESTAMP" AND
"A"."LAST_DDL_TIME"="B"."LAST_DDL_TIME" AND "A"."CREATED"="B"."CREATED" AND
"A"."OBJECT_TYPE"="B"."OBJECT_TYPE" AND
"A"."DATA_OBJECT_ID"="B"."DATA_OBJECT_ID" AND
"A"."OBJECT_ID"="B"."OBJECT_ID" AND "A"."SUBOBJECT_NAME"="B"."SUBOBJECT_NAME
" AND "A"."OBJECT_NAME"="B"."OBJECT_NAME" AND "A"."OWNER"="B"."OWNER")
Note
-----
- dynamic sampling used for this statement
统计信息
------------------------------------------------------
1801 recursive calls
0 db block gets
1407 consistent gets
0 physical reads
0 redo size
418 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
17 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> set autot off
SQL> truncate table a;
表被截断。
已用时间: 00: 00: 00.43
SQL> truncate table b;
表被截断。
已用时间: 00: 00: 00.04
SQL> drop table a;
表已删除。
已用时间: 00: 00: 00.57
SQL> drop table b;
表已删除。
已用时间: 00: 00: 00.06
SQL> create table a (id number ,name varchar2(10)) ;
表已创建。
已用时间: 00: 00: 00.10
SQL> insert into a values (1,'aa');
已创建 1 行。
已用时间: 00: 00: 00.00
SQL> insert into a values (2,'bb');
已创建 1 行。
已用时间: 00: 00: 00.00
SQL> create table b as select * from a ;
表已创建。
已用时间: 00: 00: 00.29
SQL> set autot on
SQL> select count(*) from a natural join b;
COUNT(*)
----------
2 ----结果是正确的
已用时间: 00: 00: 00.03
执行计划
----------------------------------------------------------
Plan hash value: 1397777030
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 40 | 7 (15)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 40 | | |
|* 2 | HASH JOIN | | 1 | 40 | 7 (15)| 00:00:01 |
| 3 | TABLE ACCESS FULL| A | 2 | 40 | 3 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL| B | 2 | 40 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
--------------------------------------------------
2 - access("A"."NAME"="B"."NAME" AND "A"."ID"="B"."ID")
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
103 recursive calls
0 db block gets
34 consistent gets
5 physical reads
0 redo size
419 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
4 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> truncate table a;
表被截断。
已用时间: 00: 00: 00.25
SQL> truncate table b;
表被截断。
已用时间: 00: 00: 00.01
SQL> drop table a;
表已删除。
已用时间: 00: 00: 00.28
SQL> drop table b;
表已删除。
已用时间: 00: 00: 00.06
SQL> create table b as select * from all_objects where rownum < 1000;
表已创建。
已用时间: 00: 00: 00.87
SQL> drop table a;
表已删除。
已用时间: 00: 00: 00.35
SQL> create table a as select * from b; a和b 两个表是一样的。
表已创建。
已用时间: 00: 00: 00.25
SQL> select * from a
2 minus
3 select * from b;
未选定行
已用时间: 00: 00: 00.03
SQL> select count(*) from a;
COUNT(*)
----------
999
已用时间: 00: 00: 00.01
SQL> select count(*) from b;
COUNT(*)
----------
999
已用时间: 00: 00: 00.01
SQL> set autot on
SQL> select count(*) from a natural join b;
COUNT(*)
----------
0 ---应该返回999行，但是结果是0 显然这是一个bug
已用时间: 00: 00: 00.01
执行计划
----------------------------------------------------------
Plan hash value: 1397777030
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 316 | 13 (8)| 00:00:01 |
| 1 | SORT AGGREGATE | | 1 | 316 | | |
|* 2 | HASH JOIN | | 1 | 316 | 13 (8)| 00:00:01 |
| 3 | TABLE ACCESS FULL| A | 999 | 154K| 6 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL| B | 999 | 154K| 6 (0)| 00:00:01 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("A"."EDITION_NAME"="B"."EDITION_NAME" AND
"A"."NAMESPACE"="B"."NAMESPACE" AND "A"."SECONDARY"="B"."SECONDARY" AND
"A"."GENERATED"="B"."GENERATED" AND "A"."TEMPORARY"="B"."TEMPORARY" AND
"A"."STATUS"="B"."STATUS" AND "A"."TIMESTAMP"="B"."TIMESTAMP" AND
"A"."LAST_DDL_TIME"="B"."LAST_DDL_TIME" AND "A"."CREATED"="B"."CREATED"
AND "A"."OBJECT_TYPE"="B"."OBJECT_TYPE" AND
"A"."DATA_OBJECT_ID"="B"."DATA_OBJECT_ID" AND
"A"."OBJECT_ID"="B"."OBJECT_ID" AND
"A"."SUBOBJECT_NAME"="B"."SUBOBJECT_NAME" AND
"A"."OBJECT_NAME"="B"."OBJECT_NAME" AND "A"."OWNER"="B"."OWNER")
Note
-----
- dynamic sampling used for this statement
统计信息
----------------------------------------------------------
582 recursive calls
0 db block gets
168 consistent gets
0 physical reads
0 redo size
418 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
6 sorts (memory)
0 sorts (disk)
1 rows processed
SQL> conn scott/yang
已连接。
SQL> select ename ,dname from emp natural join dept;
ENAME DNAME
---------- -----------
CLARK ACCOUNTING
KING ACCOUNTING
MILLER ACCOUNTING
JONES RESEARCH
FORD RESEARCH
ADAMS RESEARCH
SMITH RESEARCH
SCOTT RESEARCH
WARD SALES
TURNER SALES
ALLEN SALES
JAMES SALES
BLAKE SALES
MARTIN SALES
-------结果是正确的！
已选择14行。
执行计划
----------------------------------------------------------
Plan hash value: 844388907
----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 14 | 308 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN | | 14 | 308 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 52 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 14 | 126 | 4 (25)| 00:00:01 |
| 5 | TABLE ACCESS FULL | EMP | 14 | 126 | 3 (0)| 00:00:01 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("EMP"."DEPTNO"="DEPT"."DEPTNO")
filter("EMP"."DEPTNO"="DEPT"."DEPTNO")
统计信息
---------------------------------------------------
0 recursive calls
0 db block gets
11 consistent gets
0 physical reads
0 redo size
674 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
14 rows processed
---贴出执行计划，只是想看看 natural join 会走什么样的计划。
重点是这个对于all_objects 构造出的表使用natural join 查询时 没有给出正确的结果。。