When is swapping UNION for UNION ALL safe?

Question

Accepted Answer

UNION must sort or hash the combined result to find and remove duplicates, which costs extra CPU and memory. UNION ALL simply concatenates the two results without any deduplication step, making it faster. The trade-off is correctness: if duplicates exist and you do not want them, UNION ALL gives you the wrong result. A common safe case is combining two queries that select from non-overlapping date ranges or mutually exclusive filter conditions.