Warning in query plan “Cardinality Estimate”

Question

create table T(ID int identity primary key)
insert into T default values
insert into T default values

go

select cast(ID as varchar(10)) as ID
from T
where ID = 1

The query above has a warning in the query plan.

<Warnings>
  <PlanAffectingConvert ConvertIssue="Cardinality Estimate" Expression="CONVERT(varchar(10),[xx].[dbo].[T].[ID],0)" />
</Warnings>

Why does it have the warning?

How could a cast in the field list affect the cardinality estimate?

Answer 1

From New "Type Conversion in Expression....." warning in SQL2012 ,to noisy to practical use

I see what you mean. While I agree that this is noise in most cases, it is low priority for us to fix. We will look at it if we get more feedback. For now I have closed this by design.

I will provide the boring answer until someone comes along with a better one.

Why does it have the warning?

Speculation on my part.
There is a cast on a column that is used in the where clause which make statistics of that column interesting. A change of datatype makes the statistics no good so lets warn about that in case the value from the field list might end up to be used somewhere.

How could a cast in the field list affect the cardinality estimate?

It can't unless it is the field list in a derived table.