Simple paramaterization occurs when the optimizer determines that a query would benefit from a reusable plan, so it takes the hard coded values and converts them to a parameter. Great stuff. But…

Let’s take this example. Here’s a very simple query:

SELECT ct.*
FROM    Person.ContactType AS ct
WHERE   ct.ContactTypeID = 7;

This query results in simple parameterization and we can see it in the SELECT operator of the execution plan:

We can also see the parameter that was defined in use in the predicate of the seek operation:

Hang on.

Who the heck put the wrong data type in there that’s causing an implicit conversion? The query optimizer did it. Yeah. Fun stuff. If I change the predicate value to 7000 or 700000 I’ll get two more plans and I can see them all by querying the cache. But, here’s a fun little bit. I just searched the XML of the plan, the data type isn’t stored anywhere that I can see. But, if you query the plan cache, look what the text of the query is:

(@1 tinyint)SELECT [ct].* 
FROM [Person].[ContactType] [ct] 
WHERE [ct].[ContactTypeID]=@1

Now, in this case, this type of conversion doesn’t hurt performance in any way. We’re still getting an index seek. I also ran some tests with strings. The optimizer was setting them to VARCHAR(8000) and I got an implicit conversion that, again, didn’t hurt performance. But, I wouldn’t be completely shocked, if, at some point, that lead to scans. What I find it really interesting is, that the data type isn’t stored with the plan. I fully expected it to be there.

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