Question

在sp_executesql中，select语句用于连接变量本身和临时表中列的值。

当我将索引应用于此临时表并在上述select语句中使用排序时，这会产生奇怪的结果。

仅当临时表的行数大于50行时才会发生这种情况。

我不喜欢粘贴大代码示例，但我无法进一步减少它。

如果@maxjob＆gt; = 8，则生成的@htmllog包含67个字符。这是一个意想不到的结果。

如果@maxjob＆lt; 8，然后生成的@htmllog总是包含超过67个字符。这是预期的结果。

此外，

当我从#acl_log表中删除索引idx_key时，当@maxjob＆gt; = 8时问题就消失了。要么当我从@executesql_sql中删除'order by [key] asc'时，问题也会消失。

为什么？

declare @logtable as varchar(max)
set @logtable = '#acl_log'

if (OBJECT_ID('[tempdb]..#acl_log')) is not null
    drop table #acl_log

create table #acl_log(
    [key]       int             identity,
    [message]   nvarchar(max)   not null,
    index idx_key ([key])
)

declare @job as int
declare @maxjob as int

set @job = 0
set @maxjob = 8

while (@job < @maxjob + 1)
begin
    insert into #acl_log([message])
    values
        ('Internet Explorer is currently running without add-ons')
        ,('All Internet Explorer add-ons, such as ActiveX controls or toolbars, are turned off. Some webpages might not display correctly.')
        ,('To continue to your home page, click the Home button.')
        ,('To browse using add-ons, close Internet Explorer and then start it again.')
        ,('Forward Arrow  Check for the latest Windows updates. ')
        ,('Question Icon How do browser add-ons affect my browsing experience? ')

    set @job = @job + 1
end

declare @executesql_sql as nvarchar(max)
declare @executesql_param as nvarchar(max)

declare @htmllog as varchar(max)
set @executesql_sql = '
    set @htmllog = ''''
    select @htmllog = @htmllog + [message]
    from ' + @logtable + '
    order by [key] asc'
set @executesql_param = '@htmllog varchar(max) output'

exec master..sp_executesql @executesql_sql, @executesql_param, @htmllog = @htmllog output

select len(@htmllog), @htmllog

Answer 1

聚合字符串连接的行为未定义，因为它根据Microsoft在this connect item中的注释而依赖于计划：

即使没有ORDER BY，我们也不保证@var = @var + 将为任何语句生成连接值这会影响多行。表达式的右侧可以在查询执行期间进行一次或多次评估我所说的行为取决于计划。

一种解决方法是FOR XML子句：

DECLARE @logtable AS varchar(MAX);
SET @logtable = '#acl_log';

IF (OBJECT_ID('[tempdb]..#acl_log')) IS NOT NULL
    DROP TABLE #acl_log;

CREATE TABLE #acl_log(
    [key]       int             IDENTITY,
    [message]   nvarchar(max)   not null,
    INDEX idx_key ([key])
);

DECLARE @job as int;
DECLARE @maxjob as int;

SET @job = 0;
SET @maxjob = 8;

WHILE (@job < @maxjob + 1)
BEGIN
    INSERT INTO #acl_log([message])
    VALUES
        ('Internet Explorer is currently running without add-ons')
        ,('All Internet Explorer add-ons, such as ActiveX controls or toolbars, are turned off. Some webpages might not display correctly.')
        ,('To continue to your home page, click the Home button.')
        ,('To browse using add-ons, close Internet Explorer and then start it again.')
        ,('Forward Arrow  Check for the latest Windows updates. ')
        ,('Question Icon How do browser add-ons affect my browsing experience? ');

    SET @job = @job + 1;
END

DECLARE @executesql_sql AS nvarchar(MAX);
DECLARE @executesql_param AS nvarchar(MAX);

DECLARE @htmllog AS varchar(max);
SET @executesql_sql = '
    SET @htmllog = (SELECT [message]
    FROM ' + @logtable + '
    ORDER BY [key]
    FOR XML PATH(''''), TYPE).value(''.'', ''varchar(MAX)'');';

set @executesql_param = '@htmllog varchar(max) OUTPUT';

EXEC master..sp_executesql @executesql_sql, @executesql_param, @htmllog = @htmllog output;

SELECT LEN(@htmllog), @htmllog;
GO

向临时表添加索引会导致单行结果集

1 个答案: