Oracle UNION Tips

The UNION operator is great for merging the results of multiple queries that return similar rowsets, essentially executing each query separately and merging the results together into a single result set.  This is especially useful for merging remote tables using database links:

select * from fact_table@new_york
union
select * from fact_table@denver;

Whenever we need to address multiple tables in a single operation and we know that there will be no duplicate rows, we can use the UNION ALL statement to merge the tables together, as follows:

select * from fact_table_1_2011
union all
select * from fact_table_2_2011
union all
select * from fact_table_3_2011
order by
    order_year,
    order_month;

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.

 However, there are several alternatives to the union SQL operator:

1. Use UNION ALL:  The UNION ALL may be faster when you don’t mind the possibility of having duplicate rows in the result set.
   
   
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 manually.  This is slow and inefficient.
   
   
4. In versions, 10g and beyond, use the MODEL clause can simulate the behavior of the UNION clause.
   
   
5. Use a scalar subquery.  Here we see a scalar subquery equivalent to a UNION operator:
   
   select
   select col1, col2, col3 from Table_1 q1,
   select col1, col2, col3 from Table_2 q2
   from  dual;
   
   
6. Use an in-line view:  Starting in Oracle 11g release 2, the optimizer starts rewriting UNION ALL queries with the Join Factorization transformation (JFT) that rewrites some UNION ALL queries into in-line views.
   
   
7.  Re-write the UNION using a 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;

When tuning alternatives to UNION or UNION ALL, you always start by comparing and examining the execution plans, and tune each alternative independently, using the SQL*Plus “set timing on” to compares real run times.  See here, the ways to tune SQL queries by changing execution plans.

For complete details on tuning UNION and UNION ALL operators, see the book “Oracle SQL Tuning: The Definitive Reference”.