SQL Server - 计算列的索引？

I'm joining to a table dozens of different times, and every time, I join (or filter) based on the results of a SUBSTRING of one of the columns (it's a string, but left-padded with zeros, and I don't care about the last four digits). As a result, even though this column is indexed and my query would use the index, it does a table scan because the SUBSTRING itself isn't indexed, so SQL Server has to calculate it for every row before as it's joining.I'm joining to a table dozens of different time