1

我得到了一个简单的任务：为表中的每个唯一 ID 选择“id”、最小值和最大值。所以我写得很简单group by，但查询需要很长时间才能执行（30-60 秒）

SELECT     CHPDataElement.DataElementID, MIN(CHPDataElementData.UTCDataTime) AS MinDataTime, MAX(CHPDataElementData.UTCDataTime) AS MaxDataTime
FROM         CHPDataElement INNER JOIN
                      CHPDataElementData ON CHPDataElement.DataElementID = CHPDataElementData.DataElementID
GROUP BY CHPDataElement.DataElementID

order by 
CHPDataElement.DataElementID

于是我开始着手改进。并提出了在 0.3-0.5 秒内返回相同数据的简单迭代。

declare @result table
(
    DataElementID int,
    MinDataTime datetime NULL,
    MaxDataTime datetime null
)

declare @currentID int
declare @nextID int
declare @time datetime

insert into @result (DataElementID, MinDataTime, MaxDataTime)
select DataElementID,null,null from CHPDataElement
order by DataElementID

select top 1 @currentID=DataElementID from @result 

while @currentID is not null
begin
    print @currentID


    select top 1 @time=UTCDataTime  from CHPDataElementData
    where DataElementID = @currentID 
    order by UTCDataTime asc

    update @result set MinDataTime = @time
    where DataElementID = @currentID

    select top 1 @time=UTCDataTime  from CHPDataElementData
    where DataElementID = @currentID 
    order by UTCDataTime desc

    update @result set MaxDataTime = @time
    where DataElementID = @currentID



    set @nextID = null
    select top 1 @nextID=DataElementID from @result where DataElementID > @currentID
    set @currentID = @nextID
end

select * from @result

谁能解释为什么“分组依据”与第二个查询相比效率如此之低？

4

2 回答 2

0

给 CHPDataElementData 一个 DataElementID 上的索引。

于 2013-04-30T09:20:20.243 回答
0

CHPDataElementData为on添加索引DataElementID, UTCDateTime，

CREATE NONCLUSTERED INDEX IX_CHPDataElementData_DataElementID_UTCDataTime
    ON CHPDataElementData
(
    DataElementID ASC,
    UTCDataTime ASC
)

然后使用这个语句，

SELECT
            CHPDataElementData.DataElementID,
            MIN(CHPDataElementData.UTCDataTime) AS MinDataTime,
            MAX(CHPDataElementData.UTCDataTime) AS MaxDataTime
    FROM
            CHPDataElementData
    GROUP BY
            CHPDataElementData.DataElementID;
于 2013-04-30T10:31:32.167 回答