SQL Server 在 nvarchar 字符串中搜索时使用高 CPU [英] SQL Server uses high CPU when searching inside nvarchar strings

问题描述

看看下面的例子.它表明在 unicode 字符串 (nvarchar) 中搜索几乎是在 varchar 字符串中搜索的八倍.与隐式转换相当.寻找对此的解释.或者更有效地在 nvarchar 字符串中搜索的方法.

Check out the following example. It shows that searching within a unicode string (nvarchar) is almost eight times as bad as searching within a varchar string. And on par with implicit conversions. Looking for an explanation for this. Or a way to search within nvarchar strings more efficiently.

use tempdb
create table test
(
    testid int identity primary key,
    v varchar(36),
    nv nvarchar(36),
    filler char(500)
)
go

set nocount on
set statistics time off
insert test (v, nv)
select CAST (newid() as varchar(36)),
    CAST (newid() as nvarchar(36))
go 1000000

set statistics time on
-- search utf8 string
select COUNT(1) from test where v like '%abcd%' option (maxdop 1)
-- CPU time = 906 ms,  elapsed time = 911 ms.

-- search utf8 string using unicode (uses convert_implicit)
select COUNT(1) from test where v like N'%abcd%' option (maxdop 1)
-- CPU time = 6969 ms,  elapsed time = 6970 ms.

-- search unicode string
select COUNT(1) from test where nv like N'%abcd%' option (maxdop 1)
-- CPU time = 6844 ms,  elapsed time = 6911 ms.

推荐答案

正在寻找对此的解释.

Looking for an explanation for this.

NVarchar 是 16 位，Unicode 比较规则比 ASCII 复杂得多 - 同时支持的各种语言的特殊字符需要引用更多处理.

NVarchar is 16 bit and Unicode comparison rules are a lot more complicated than ASCII - special chars for the various languages that are supported at the same time require quote some more processing.

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