说明
1、在MySQL对于join操作的处理过程中，join buffer是一个重要的概念。
2、是MySQL对于table join的一个重要的优化手段。虽然这个概念实现并不复杂，但是这个是实现MySQL join连接优化的一个重要方法，在连接的时候可以极大提高join查询的效率。
实例
Table name Type
t1 range
t2 ref
t3 ALL
The join is then done as follows:
- While rows in t1 matching range
- Read through all rows in t2 according to reference key
- Store used fields from t1, t2 in cache
- If cache is full
- Read through all rows in t3
- Compare t3 row against all t1, t2 combinations in cache
- If row satisfies join condition, send it to client
- Empty cache
- Read through all rows in t3
- Compare t3 row against all stored t1, t2 combinations in cache
- If row satisfies join condition, send it to client 
