小编给大家分享一下ORACLE MYSQL中join 字段类型不同索引失效的情况有哪些，相信大部分人都还不怎么了解，因此分享这篇文章给大家参考一下，希望大家阅读完这篇文章后大有收获，下面让我们一起去了解一下吧！
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 * from testjoin1 a join testjoin2 b on a.id=b.id ;
ID NAME ID NAME
---------- -------------------- -------------------- --------------------
1 gaopeng 1 gaopeng
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 `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`)
以上是“ORACLE MYSQL中join 字段类型不同索引失效的情况有哪些”这篇文章的所有内容，感谢各位的阅读！相信大家都有了一定的了解，希望分享的内容对大家有所帮助，如果还想学习更多知识，欢迎关注亿速云行业资讯频道！
