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首先常规的数据库存储引擎,一般采用b树或者B+树来实现索引的存储,因为b树呢是一种多路平衡树
用这种存储结构来存储大量数据的情况下呢,它的整体高度相比二叉树来说比较低,而对于数据库来说,所有的数据存储必然是存储在磁盘上的而磁盘io的效率事件上是很低的,特别是随机磁盘的一个情况下效率更低,所以树的高度决定磁盘io一个次数,磁盘io次数越少,那么对性能的提升就会越大,采用b树作为索引存储结构的原因,但是mysql的innoDB存储引擎里面它采用的是一种增强的b树结构也就是b+树来作为索引和数据的一个存储结构,相比较于b树的结构,B+树做了几个方面的优化
1,b+树的所有数据都存储在叶子节点,非叶子节点存储索引
2,叶子节点中的数据使用双向链表的方式进行关联
为什么做以上两个优化
1.B+树非叶子节点不存储数据,所以每一层能够存储的索引数量会增加,意味b+树在层高相同的情况下存储数据量要比b树要多,使得磁盘io次数更少
2.Mysql中,范围查询时一个比较常用的操作,而b+树的所有存储在叶子节点的数据使用了双向链表来关联,所以在查询的时候只需要查两个节点进行遍历就行,而b需要获取到所有的节点,所以b+树在范围查询效率更高。
3.在数据检索方面,由于所有的数据都存储在叶子节点,所以b+树的io次数会更加稳定一些。
4.因为叶子节点存储所有数据,所有b+树的全局扫描能更强一些,因为它只需要扫描叶子节点,但b树需要遍历整个树。