Question

Is it fair to say that it takes no time (compared to the nested SELECT) to make the second (outer) ‘SELECT’ from a result-set like this?

    SELECT some_column 
    FROM
         (
           SELECT some_column
           FROM some_table
         )
    AS _alias

Answer 1

The SQL optimizer is likely to treat that SELECT statement as if it was written:

SELECT some_column FROM some_table

So there’ll be no performance difference whatsoever. The optimizer does its best to minimize the cost of producing the answer and will rework the query you write to speed things up. Only the most naïve optimizer would evaluate the inner SELECT and save the results in a table and then run the outer SELECT on that result.