rounded card notes from AskTom

最新推荐文章于 2025-08-10 13:24:43 发布

转载最新推荐文章于 2025-08-10 13:24:43 发布 · 71 阅读

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原文链接：http://www.cnblogs.com/helloolduncle/archive/2011/11/24/2262314.html

文章标签：

#数据库

本文探讨了数据库执行计划中rounded cardinality的概念及其与original cardinality的区别，并详细对比了autotrace与explain plan的不同之处，包括变量绑定、类型转换等方面。

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Note 1:What is a rounded cardinality?

rounded card是执行计划时的预测数值

original card是对象实际的原始的数值

Note 2: auto trace和explain plan有哪些区别：

a) explain plan does not bind variable peek, a real query would.

b) explain plan sees all binds as varchar2's - in real life they are numbers, dates, varchar2's and 
so on.  Hence, implicit conversions may or may not be observed by explain plan that would be 
observed in real life.
c) explain plan does a hard parse, uses the statistics/dynamic samples that are IN PLACE RIGHT NOW. 
 A query that was run for real "a little while ago" or "a little while from now" might see 
different inputs
d) explain plan uses the current operational environment to come up with a plan.  You might be 
explaining in a FIRST_ROWS session, whereas the query was actually executed in a ALL_ROWS session 
(or any one of the optimizer parameters could be different)
e) any combination of a, b, c, d...

http://asktom.oracle.com/pls/apex/f?p=100:11:0::::P11_QUESTION_ID:63445044804318

转载于:https://www.cnblogs.com/helloolduncle/archive/2011/11/24/2262314.html