When you pass a C# string through an anonymous object, Dapper maps it to nvarchar(4000). That’s the default mapping for System.String in ADO.NET — and honestly, it makes sense from a “safe default” perspective. But if your column is varchar, SQL Server has to convert every single value in the column to nvarchar before it can compare. This is called CONVERT_IMPLICIT, and it means SQL Server can’t use your index. Full scan. Every time.
You will still find supposedly extremely sophisticated LLMs making extremely
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