ShareJoin是一个简单的跨分片Join,基于HBT的方式实现。
目前支持2个表的join,原理就是解析SQL语句，拆分成单表的SQL语句执行，然后把各个节点的数据汇集。
配置支持任意配置的A,B表，如：
A,B的dataNode相同
A,B的dataNode不同
或
insert company (id,name) values(1,'mycat');
Query OK, 1 row affected (0.08 sec)
mysql> insert company (id,name) values(2,'ibm');
Query OK, 1 row affected (0.03 sec)
mysql> insert company (id,name) values(3,'hp');
Query OK, 1 row affected (0.03 sec)
下面可以看下普通的join和sharejoin的区别
mysql> select a.*,b.id, b.name as tit from customer a,company b where a.company_id=b.id;
+----+------+------------+-------------+----+------+
| id | name | company_id | sharding_id | id | tit |
+----+------+------------+-------------+----+------+
| 3 | feng | 3 | 10000 | 3 | hp |
+----+------+------------+-------------+----+------+
1 row in set (0.03 sec)
mysql> /*!mycat:catlet=demo.catlets.ShareJoin */ select a.*,b.id, b.name as tit from customer a,company b on
a.company_id=b.id;
+----+------+------------+-------------+----+-------+
| id | name | company_id | sharding_id | id | tit |
+----+------+------------+-------------+----+-------+
| 3 | feng | 3 | 10000 | 3 | hp |
| 1 | wang | 1 | 10000 | 1 | mycat |
| 2 | xue | 2 | 10010 | 2 | ibm |
+----+------+------------+-------------+----+-------+
3 rows in set (0.05 sec)
其他两种写法
/*!mycat:catlet=demo.catlets.ShareJoin */ select a.*,b.id, b.name as tit from customer a join company b on
a.company_id=b.id;
+----+------+------------+-------------+----+-------+
| id | name | company_id | sharding_id | id | tit |
+----+------+------------+-------------+----+-------+
| 3 | feng | 3 | 10000 | 3 | hp |
| 1 | wang | 1 | 10000 | 1 | mycat |
| 2 | xue | 2 | 10010 | 2 | ibm |
+----+------+------------+-------------+----+-------+
3 rows in set (0.01 sec)
/*!mycat:catlet=demo.catlets.ShareJoin */ select a.*,b.id, b.name as tit from customer a join company b
where a.company_id=b.id;
+----+------+------------+-------------+----+-------+
| id | name | company_id | sharding_id | id | tit |
+----+------+------------+-------------+----+-------+
| 3 | feng | 3 | 10000 | 3 | hp |
| 1 | wang | 1 | 10000 | 1 | mycat |
| 2 | xue | 2 | 10010 | 2 | ibm |
+----+------+------------+-------------+----+-------+
3 rows in set (0.01 sec)
对*的支持，还可以这样写SQL 
