对PL / SQL Developer中的SQL成本感到困惑

Question

Select pn.pn_level_id From phone_number pn Where pn.phone_number='15183773646'


Select pn.pn_level_id From phone_number pn Where pn.phone_number=' 15183773646'

Do you think they are the same ? No. Basically they are not the same in the PL/SQL Devloper.

I'm wondering why the latter one's cost is less than the previous sql. Any help would be appreciated!

Answer 1

The cost is not the same because when cost is calculated planner takes into account available statistics. Statistics among other things contains values which appear most frequently in column with their frequencies. It allows planner to better estimate number of rows which will be fetched and to decide how to better get data (eg via sequential scan or by index).

In your case value 15183773646 is probably among mostly frequently appearing and that's why planner estimation is different for the query involving it (as planner has better estimation of number of such rows) as compared to other values for which it basically guesses.

Answer 2

优化程序会评估所有方法来提取数据，因为您的数据库没有pn.phone_number=' 15183773646'并且该值未存储在索引中，因此Oracle跳过了一个I / O操作，这就是第二个成本较低的原因。