Comment by wvenable
3 hours ago
For a literal/parameter that happens to be ASCII, a person might know it would fit in varchar, but the optimizer has to choose a plan that stays correct in the general case, not just for that one runtime value. By telling SQL server the parameter is a nvarchar value, you're the one telling it that might not be ASCII.
optimizer can't inspect the value? pretty dumb optimizer, then.
It's not "the value", it's "the values".
Running the optimizer for every execution of the same query is... not very optimal.