脚本如下：


mysql:
drop table testjoin1;
drop table testjoin2;
create table testjoin1(id int, name varchar(20));
create table testjoin2(id varchar(20),name varchar(20),key(id);


oracle:
drop table testjoin1;
drop table testjoin2;
create table testjoin1(id int,name varchar2(20));
create table testjoin2(id varchar(20),name varchar2(20));
create index test_id_2 on testjoin2(id);


insert into testjoin1 values(1,'gaopeng');
insert into testjoin1 values(2,'gaopeng');
insert into testjoin1 values(3,'gaopeng');
insert into testjoin1 values(4,'gaopeng');
insert into testjoin1 values(5,'gaopeng');
insert into testjoin2 values('1','gaopeng');




ORACLE中的隐士转换，
SQL> select /*+ use_nl(a b) ordered */ * from testjoin1 a join testjoin2 b on a.id=b.id ;

ID NAME IDNAME
---------- -------------------- -------------------- --------------------
1 gaopeng 1gaopeng

Execution Plan
----------------------------------------------------------
Plan hash value: 2498279186


--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
|0 | SELECT STATEMENT|| 1 |49 | 5(0)| 00:00:01 |
|1 | NESTED LOOPS || 1 |49 | 5(0)| 00:00:01 |
|2 |TABLE ACCESS FULL| TESTJOIN1 | 5 |125 | 2(0)| 00:00:01 |
|* 3 |TABLE ACCESS FULL| TESTJOIN2 | 1 |24 | 1(0)| 00:00:01 |
--------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("A"."ID"=TO_NUMBER("B"."ID")) --虽然TESTJOIN2是被驱动表由于隐士转换索引用不到

mysql> explain select * from testjoin1 a Straight_JOIN testjoin2 b on a.id=b.id ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | a | NULL| ALL | NULL | NULL | NULL| NULL |5 |100.00 | NULL|
| 1 | SIMPLE | b | NULL| ALL | id| NULL | NULL| NULL |1 |100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
2 rows in set, 3 warnings (0.00 sec)

Warning (Code 1739): Cannot use ref access on index 'id' due to type or collation conversion on field 'id' --虽然TESTJOIN2是被驱动表由于隐士转换索引用不到 possible_keys可以看出
Warning (Code 1739): Cannot use range access on index 'id' due to type or collation conversion on field 'id'
Note (Code 1003): /* select#1 */ select `test`.`a`.`id` AS `id`,`test`.`a`.`name` AS `name`,`test`.`b`.`id` AS `id`,`test`.`b`.`name` AS `name` from `test`.`testjoin1` `a` straight_join `test`.`testjoin2` `b` where (`test`.`a`.`id` = `test`.`b`.`id`)