Execution plan warnings–All that glitters is not gold
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by Dave Ballantyne
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Published on Sun, 06 Nov 2011 15:21:51 GMT
Indexed on
2011/11/11
18:07 UTC
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In a previous post, I showed you the new execution plan warnings related to implicit and explicit warnings. Pretty much as soon as i hit ’post’, I noticed something rather odd happening.
This statement :
select top(10)
SalesOrderHeader.SalesOrderID,
SalesOrderNumber
from Sales.SalesOrderHeader
join Sales.SalesOrderDetail
on SalesOrderHeader.SalesOrderID
= SalesOrderDetail.SalesOrderID
Throws the “Type conversion may affect cardinality estimation” warning.
Ive done no such conversion in my statement why would that be ? Well, SalesOrderNumber is a computed column , “(isnull(N'SO'+CONVERT([nvarchar](23),[SalesOrderID],0),N'*** ERROR ***'))”, so thats where the conversion is.
Wait!!! Am i saying that every type conversion will throw the warning ? Thankfully, no. It only appears for columns that are used in predicates ,even if the predicate / join condition is fine , and the column is indexed ( and/or , presumably has statistics).
Hopefully , this wont lead to to many wild goose chases, but is definitely something to bear in mind. If you want to see this fixed then upvote my connect item here.
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