BE VERY CAREFUL HERE, from what you posted I suspect that @Start_Date DATETIME and @Part_Type_MP are inbound parameters to your stored proc. You have created a huge hole here that is ripe for sql injection. You allow the user to pass you a string and then you execute it. This is the classic scenario for sql injection. Consider what would happen if somebody passed "'); drop table Purchasing_v_Line_Item_e;--". It would throw a syntax error for the first statement, then drop your purchasing table.
You should parameterize your query and use sp_executesql instead of a simple exec.
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