SQL Alternative to UNION clause

Answer:  A UNION is highly optimized and really fast, except in cases where one query finishes long before the other, and Oracle must wait to get the whole result set before starting sorting.  

There are several alternatives to the union SQL operator:

1. Use UNION ALL.
    
2. Execute each SQL separately and merge and sort the result sets within your program!  Sometimes, an external sort may be faster.
   
   
3. Join the tables.  This can be kinda slow, but it is an alternative to using UNION.
   
   
4. In versions, 10g and beyond, explore the MODEL clause.
   
   
5. Use a scalar subquery.
   
   select
   select col1, col2, col3 from Table_1 q1,
   select col1, col2, col3 from Table_2 q2
   from  dual;
   
   
6. Try UNION using FULL OUTER JOIN with the NVL function:  It is suggested that this has faster performance than the UNION operator.
   
   select
       empno,
       ename,
        nvl(dept.deptno,emp.deptno) deptno, dname
   from
       emp
   full outer join
   dept
   on
       (emp.deptno = dept.deptno)
   order by 1,2,3,4;

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You start by examining the execution plans, and tune them independently.

A union simply executes two independent queries and merges the result sets.

There is one tuning trick you can use if the union contains an ORDER BY or GROUP BY ...

I just came out with a book that discusses this in great detail:  Advanced Oracle SQL Tuning: The Definitive Reference.