为了更加深入左外连接，我们做一些測试，外连接的写法有几种形式，我们能够通过10053跟踪到终于SQL转换的形式。

--初始化数据

create table A

(

  id number,

  age number

);

create table b

(

  id number,

  age number

);

insert into A values(1,10);

insert into A values(2,20);

insert into A values(3,30);

insert into B values(1,10);

insert into B values(2,20);

commit;

--用10053找到终于转换后的SQL

alter session set session_cached_cursors =0;

alter session set events '10053 trace name context forever, level  1';

explain plan for select * from A left join B on A.id = B.id and A.age > 5;

explain plan for select * from A left join B on A.id = B.id WHERE A.age > 5;

explain plan for select * from A left join B on A.id = B.id and b.age > 5;

explain plan for select * from A left join B on A.id = B.id where b.age > 5;

alter session set events '10053 trace name context off' ;





select * from A left join B on A.id = B.id and A.age > 5;

        ID        AGE         ID        AGE

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

         1         10          1         10

         2         20          2         20

         3         30

--Final query after transformations:

SELECT "A"."ID" "ID", "A"."AGE" "AGE", "B"."ID" "ID", "B"."AGE" "AGE"

  FROM "GG_TEST"."A" "A", "GG_TEST"."B" "B"

 WHERE "A"."ID" = "B"."ID"(+)

   AND "A"."AGE" > CASE WHEN("B"."ID"(+) IS NOT NULL) THEN 5 ELSE 5 END





select * from A left join B on A.id = B.id WHERE A.age > 5;

        ID        AGE         ID        AGE

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

         1         10          1         10

         2         20          2         20

         3         30

--Final query after transformations:

SELECT "A"."ID" "ID", "A"."AGE" "AGE", "B"."ID" "ID", "B"."AGE" "AGE"

  FROM "GG_TEST"."A" "A", "GG_TEST"."B" "B"

 WHERE "A"."AGE" > 5

   AND "A"."ID" = "B"."ID"(+);





select * from A left join B on A.id = B.id and b.age > 5; 

        ID        AGE         ID        AGE

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

         1         10          1         10

         2         20          2         20

         3         30  

--Final query after transformations:

SELECT "A"."ID" "ID", "A"."AGE" "AGE", "B"."ID" "ID", "B"."AGE" "AGE"

  FROM "GG_TEST"."A" "A", "GG_TEST"."B" "B"

 WHERE "A"."ID" = "B"."ID"(+)

   AND "B"."AGE"(+) > 5



--这样的形式你能够看到外连接失效，CBO还是很聪明的

select * from A left join B on A.id = B.id where b.age > 5;

        ID        AGE         ID        AGE

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

         1         10          1         10

         2         20          2         20

--Final query after transformations:

SELECT "A"."ID" "ID", "A"."AGE" "AGE", "B"."ID" "ID", "B"."AGE" "AGE"

  FROM "GG_TEST"."A" "A", "GG_TEST"."B" "B"

 WHERE "B"."AGE" > 5

   AND "A"."ID" = "B"."ID";