You are going to see a performance drop, because you are going to have to union the two tables together and then query on those just to be able to accomplish an artificial partition of the data.
SQL Server is NOT supposed to give up memory after each query. Memory allocation is an expensive operation. A SQL Server will give up memory if something else on the machine needs it. Since nothing else needs it, SQL Server is going to keep it, because it will probably need the memory again at a later time. If the SQL Server gave up memory after every query, you would quickly have a system that would spend more than half of the time doing nothing but allocating and deallocating memory at which point there would be very little purpose to even trying to run something on it.
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