The optimizer cost model relies on statistics collected about the objects involved in a query, and the database and host where the query runs.

Statistics are critical to the optimizer's ability to pick the best execution plan for a SQL statement.

Optimizer statistics include the following:

Table statistics

Number of rows
Number of blocks
Average row length

Column statistics

Number of distinct values (NDV) in a column
Number of nulls in a column
Data distribution (histogram)
Extended statistics

Index statistics

Number of leaf blocks
Number of levels
Index clustering factor
System statistics

I/O performance and utilization
CPU performance and utilization
As shown in Figure 10-1, the database stores optimizer statistics for tables, columns, indexes, and the system in the data dictionary. You can access these statistics using data dictionary views.

十余年的临河网站建设经验，针对设计、前端、开发、售后、文案、推广等六对一服务，响应快，48小时及时工作处理。网络营销推广的优势是能够根据用户设备显示端的尺寸不同，自动调整临河建站的显示方式，使网站能够适用不同显示终端，在浏览器中调整网站的宽度，无论在任何一种浏览器上浏览网站，都能展现优雅布局与设计，从而大程度地提升浏览体验。创新互联建站从事“临河网站设计”,“临河网站推广”以来，每个客户项目都认真落实执行。

分享题目：IntroductiontoOptimizerStatistics
文章来源：http://myzitong.com/article/iejdhd.html