嵌套選擇內昂貴的函數

Question

我正在做一個相對複雜的CTE查詢與昂貴的用戶定義的標量函數調用。

CTE的where子句過濾大部分大表數據，並返回相對較小的結果，因此如果計算結果集，計算昂貴的字段不是問題。

如果不評估昂貴的列，如果沒有被CTE子查詢中的任何謂詞所觸及，SQL服務器就會很好地工作，它僅對結果集進行評估。

問題是，在這種情況下，我可以依賴優化程序的這種優良行爲，還是在計劃重建時可能出錯？

這是一個測試代碼。

use tempdb; 
go 
/****** Object: UserDefinedFunction [dbo].[expensive] Script Date: 01/15/2010 18:43:06 ******/ 
IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[expensive]') AND type in (N'FN', N'IF', N'TF', N'FS', N'FT')) 
DROP FUNCTION [dbo].[expensive] 

go 
-- used to model expensive user defined function. 
-- inexpensive in case of @i = 1 
create function dbo.expensive(@i int) 
returns int 
as 
begin 
if(@i = 1) begin return @i; -- inexpensive in this case 
end; 
declare @N bigint 
declare @ret bigint 
set @N = 17; -- will generate a set of 2^N 

declare @tab table(num int); 

with gig as 
(
    select 1 as num 
    union all 
    select g.num + 1 as num from gig g 
    where g.num < @N 
    union all 
    select g.num + 1 as num from gig g 
    where g.num < @N 
) 
select @ret = count(num) from gig; 
return @ret; 
end 

go 

declare @tab table(i int); 

insert into @tab select 1 union select 2 union select 3; 

select CURRENT_TIMESTAMP; 

with source as 
(
-- some really complex stuff that has an expensive calcutated scalar 
select c.i, c.caclulated from @tab t 
join 
(select i, dbo.expensive(i) as caclulated from @tab) as c 
on t.i = c.i 
) 
select * from source where 
i = 1; -- this query is inexpensive, because do not touch anything but 1 

select CURRENT_TIMESTAMP; 

-- this one is expensive 
select dbo.expensive(2) 

select CURRENT_TIMESTAMP; 

Answer 1

你永遠不會知道。你可以做的一件事是使用計劃指南來幫助優化器保持正確的路徑。見Optimizing Queries in Deployed Applications by Using Plan Guides。