HANA数据库为何如此之快
原文标题是How SAP HANA Is Such a Fast Database，不过作者的观点是HANA的快主要源自硬件的发展，而且HANA并非适合所有的应用场景。 不过我关注的恰好是结论之外的部分。 数据访问模式的变化 存储硬件的提升，从物理磁盘到SSD, 内存，相应的数据库查询方式
原文标题是How SAP HANA Is Such a Fast Database，不过作者的观点是HANA的快主要源自硬件的发展，而且HANA并非适合所有的应用场景。
不过我关注的恰好是结论之外的部分。
数据访问模式的变化
存储硬件的提升，从物理磁盘到SSD, 内存，相应的数据库查询方式也发生了变化。
当数据库使用传统的物理磁盘，如果在一个具有10列的表中读取3列，你也必须读取全部10列。
如果表为100M，100万行，那么必须读取整个表来完成查询，在涉及多表Join时，情况更加严重。
如果数据库存储在SSD或内存这些随机访问型存储上，你需要3列，就只需读取3列。
数据交换的消除
传统的存于磁盘的数据库需要花费大量的时间来进行Swapping，即将数据读入缓存，处理后随后又将数据写回磁盘，周而复始，生生不息, :)。
而内存数据库则消除了这种数据交换(Swapping)，因为所有的数据都在内存中操作，需要存储的唯一目的就是为了数据持久化和备份。
其实这解释了即使将数据全部Cache到内存，仍然不如内存数据库快的原因，无论是列式数据库如HANA, 还是行式数据库如TimesTen。
结论
列式数据库有速度的优势，但不是万能的优势，尽管SAP希望将HANA作为一体适用的平台，不过One Size Does Not Fit All, 此数据库的原理来说，HANA非常适合BW这样的分析性应用，对于ERP这样的OLTP的应用，我们还是拭目以待，看看S/4 HANA的作为吧。
参考
- How SAP HANA Is Such a Fast Database - SCM Focus’ SAP HANA Blog
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