Why is SQL Server using a clustered index scan for a self referencing FK cascade delete
It needs to validate that the row you are trying to delete is not a parent of an existing row.

You don't have an index on ParentTestId.

So it must do the scan.

CREATE NONCLUSTERED INDEX ix ON  [dbo].[Test](ParentTestId)


Then you see a seek.

BTW: The 20% estimated cost of the scan is likely to be an underestimate in this case. 

The FK validation is under a left semi join and SQL Server costs it as though only a partial scan will be needed and it will find a matching row and the delete will fail.

Presumably the rows you are actually deleting will succeed more often than not and so a full scan will be required in order to validate that there are no conflicting rows.

Using trace flag 4138 to turn off row goals

DELETE FROM dbo.Test
WHERE  TestId = 200 
OPTION (querytraceon 4138 )


The re-costed plan shows the CI scan at 100% rather than 20% (as it now assumes a full scan will be needed)



This difference in estimated cost is sufficient for the missing index suggestion to show up.

The costs shown in this plan are still not very representative however. You might notice that they add up to 219%.

Also the overall plan cost of the queries with and without the trace flag are both identical at 0.0168268. The full CI scan ought, in fact, to be costed at 0.152373 (0.0485075 + 0.103866)



but it seems to be capped at no more than the original plan cost (and the overall plan cost doesn't get adjusted upwards either hence incorrect percentages)