我正在通過CTE更新自加入的結果表。當選擇查看更新的預期結果時,它可以在16秒內返回結果。來自CTE的更新
但是,當我使用相同的語法來嘗試實際更新基礎表時,它非常慢。
對於我的生活,我無法弄清楚爲什麼如此戲劇性的放緩。我經常通過CTE更新表格,與同等選擇相比,它通常更加快速和合理。
我已經在底層表上嘗試過和沒有PrimaryKey/Clustered索引,它沒有區別。
該聯接位於計算列上,因此無法建立索引。
如果SELECT和UPDATE之間的時間差異是兩倍,那麼它就不是問題。這裏的問題是從選擇到更新的時間增加的巨大程度。對於選擇結果爲1323行的16秒,更新其中2行需要59秒,更新4需要1分19秒,更新6需要1分39秒(因此看起來每個附加行增加10秒)。
任何人都可以爲我闡明這一點,並提出一種方法來加速?
這裏的示例代碼:
;WITH CTE AS (SELECT
DENSE_RANK() OVER (Order by
col1,
col2,
col3) SetID,
COUNT(*) OVER (partition by
col1,
col2,
col3) DupsInSet,
row_number() OVER (PARTITION BY
col1,
col2,
col3
ORDER BY
col4 desc) RowInSet,
COUNT(col4) OVER (partition by
col1,
col2,
col3) NonNull,
*
FROM mytable)
在16秒後--The下完成,並返回1323行
select b.col4,a.*
from cte a
join cte b on b.SetID=a.SetID
where a.DupsInSet>1
and a.NonNull>0
and b.RowInSet=1
and a.RowInSet>1
and b.col4 is not null
and a.col4 is null
從這個--Updating用於運行這麼長時間,我甚至沒有讓它完成 - 作爲一項測試,我將更新限制在前2名。然後這花了59秒來更新2行
UPDATE TOP(2) a
SET a.col4=b.col4
from cte a
join cte b on b.SetID=a.SetID
where a.DupsInSet>1
and a.NonNull>0
and b.RowInSet=1
and a.RowInSet>1
and b.col4 is not null
and a.col4 is null
爲UPDATE https://www.brentozar.com/pastetheplan/?id=Ske_In3_Z
更新選擇https://www.brentozar.com/pastetheplan/?id=ByItSnhuW
實際執行計劃實際執行計劃:
以從SqlZim它跑了4分鐘的建議沒有完成,在這一點上,我停止了。
然而,隨着
- 在模式的改變(謝謝SqlZim)從幾個VARCHAR(MAX)列到VARCHAR(?)在哪裏?是MAX(LEN(列))
和
- 一些修改SqlZims建議的查詢,則更新能夠在23秒運行!...其大約是3,300倍:)
這裏是最後一個查詢(除非有人可以使其工作,而不必列出所有darn列,例如加入對SETID):
(注意,熱膨脹係數仍用於從600K +行下過濾原始表)
;WITH CTE AS (SELECT
COUNT(*) OVER (partition by
[col1]
,[col2]
,[col3]
,[col4]
,[col5]
,[col6]
,[col7]
,[col8]
,[col9]
,[col10]
,[col11]
,[col12]
,[col13]
,[col14]
,[col15]
,[col16]
,[col17]
,[col18]
,[col19]
,[col20]
,[col21]
,[col22]
,[col23]) DupsInSet,
COUNT(col24) OVER (partition by
[col1]
,[col2]
,[col3]
,[col4]
,[col5]
,[col6]
,[col7]
,[col8]
,[col9]
,[col10]
,[col11]
,[col12]
,[col13]
,[col14]
,[col15]
,[col16]
,[col17]
,[col18]
,[col19]
,[col20]
,[col21]
,[col22]
,[col23]) NonNull,
*
FROM mytable
)
update a
set a.col24 = b.col24
from cte a
cross apply (
select top 1 i.col24
from cte i
where (i.col1=a.col1 OR (i.col1 is null AND a.col1 is null))
and (i.col2=a.col2 OR (i.col2 is null AND a.col2 is null))
and (i.col3=a.col3 OR (i.col3 is null AND a.col3 is null))
and (i.col4=a.col4 OR (i.col4 is null AND a.col4 is null))
and (i.col5=a.col5 OR (i.col5 is null AND a.col5 is null))
and (i.col6=a.col6 OR (i.col6 is null AND a.col6 is null))
and (i.col7=a.col7 OR (i.col7 is null AND a.col7 is null))
and (i.col8=a.col8 OR (i.col8 is null AND a.col8 is null))
and (i.col9=a.col9 OR (i.col9 is null AND a.col9 is null))
and (i.col10=a.col10 OR (i.col10 is null AND a.col10 is null))
and (i.col11=a.col11 OR (i.col11 is null AND a.col11 is null))
and (i.col12=a.col12 OR (i.col12 is null AND a.col12 is null))
and (i.col13=a.col13 OR (i.col13 is null AND a.col13 is null))
and (i.col14=a.col14 OR (i.col14 is null AND a.col14 is null))
and (i.col15=a.col15 OR (i.col15 is null AND a.col15 is null))
and (i.col16=a.col16 OR (i.col16 is null AND a.col16 is null))
and (i.col17=a.col17 OR (i.col17 is null AND a.col17 is null))
and (i.col18=a.col18 OR (i.col18 is null AND a.col18 is null))
and (i.col19=a.col19 OR (i.col19 is null AND a.col19 is null))
and (i.col20=a.col20 OR (i.col20 is null AND a.col20 is null))
and (i.col21=a.col21 OR (i.col21 is null AND a.col21 is null))
and (i.col22=a.col22 OR (i.col22 is null AND a.col22 is null))
and (i.col23=a.col23 OR (i.col23 is null AND a.col23 is null))
and i.col24 is not null
order by col24 desc
) b
where a.col24 is null
and a.DupsInSet>1
and a.NonNull>0
使用[粘貼該計劃@ brentozar.com分享您的執行計劃](https://www.brentozar.com/pastetheplan/)這裏是說明:[如何使用粘貼計劃](https://www.brentozar.com/pastetheplan/instructions/)。樣本數據和期望的結果也會有所幫助。 – SqlZim
@SqlZim - 已發佈編輯以添加執行計劃的鏈接。鏈接到選擇的示例行https://ibb.co/huyQhQ – tentrade
爲什麼要從CTE更新CTE?這是什麼意思,因爲它僅在CTE創建後的第一個聲明中可用? – HLGEM