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HomeDatabaseMysql Tutorial【Oracle】-【创建索引】-创建索引的操作原理与一些体会

1、将index key的data读到cache。如果之前这部分数据未读到DB Cache,那么此时可能有db file scatter read write的等待事件。 2、对index key的data进行排序。排序的操作,如果sort_area_size或pga_aggregate_target不大的情况下,可能就会做disk sort,我们

1、将index key的data读到cache。如果之前这部分数据未读到DB Cache,那么此时可能有db file scatter read write的等待事件。

2、对index key的data进行排序。排序的操作,如果sort_area_size或pga_aggregate_target不大的情况下,可能就会做disk sort,我们知道,磁盘排序的效率要小于Cache不少,因此速度上肯定受影响,会有direct path read/write,同时也会消耗CPU time。

3、创建新的index segment(补充:block是最小的I/O单元,extent是最小的Oracle空间分配单元,segment又是由extent组成的,Oracle中表、索引、视图等等的存储都可以看做是segement),将排序的index data写到创建的index segment中。那么如果index很大,就会有许多的redo log相关等待,例如log file sync,log buffer space,log file parallel write等。


参考牛人的随笔后,


关于创建索引的一点体会:

(1)、关于利用并行度创建索引,前提是多个CPU,单CPU下用并行度创建索引,可能会造成资源的争用,dave曾经推测过可能是CPU的争用,也可能是I/O的争用,造成的结果就是比不用并行度消耗更多的时间才能创建索引。

(2)、在建大表的索引时,可以增大PGA,增大temp tablespace,因为排序通常是在PGA中进行的,防止因空间或内存不足导致需要disk排序,是最大的问题。但往往有时这些参数不让随意调整。那么nologging这种方式可能还是可以做的。


关于索引利用的一点体会:

(1)、关于第二点,index data都会进行排序,那么利用索引的这个特性,有时可以避免对表的排序操作,例如当需要查询max或min这种排序结果时,只要建立某个字段的索引,就可以避免order by的使用,因为index中的数据已经排序。

(2)、同时利用索引,有时也可以避免回表,对于select某些索引字段时,这种方式的效果更好。

关于这些方面这里谈的很少,后面找个机会单独讨论。

作者:bisal 发表于2013-8-9 14:18:39 原文链接

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【Oracle】-【创建索引】-创建索引的操作原理与一些体会

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