Also know that the optimizer, with one exception, treats a parameter (before the AS) and a variable (after the AS) differently. A parameter value is known to the optimizer when it compiles the query since the parameter has to be passed with the execution statement, while a variable value is not known (except in the case of a recompile, the one exception). The known values are used to compare against the statistics to arrive at a more specific plan for the data being passed. This process is known as parameter sniffing. A variable, because it's value is unknown, will get an average plan based on an average of the statistics (except, again, in the case of recompiles).