Optimizing Sort Operations - Oracle Database 11gR2 Performance Tuning - page 237

In step 3, we execute the same query mentioned earlier, but using the UNION ALL operator,
resulting in the following execution plan:
We can see that if we use UNION ALL instead of the UNION operator, the SORT UNIQUE
operation is not required. Remember that the two operations are similar, but the UNION ALL
returns duplicate records, and the UNION operator doesn't. When we don't have to worry
about the duplicates (or they are not required by the design), it's better to use the UNION ALL
operator to improve performance.