performance of union versus union all
UNION ALL will perform better than UNION when you're not concerned about eliminating duplicate records because you're avoiding an expensive distinct sort operation. See: SQL SERVER – Difference Between Union vs. Union All – Optimal Performance Comparison
UNION ALL always is faster, because UNION exclude duplicated entries
UNION implement internally two queries.1.SELECT which will return a dataset2.DISTINCT. Anyone who has studied database internals can easily understand that a DISTINCT clause is extremely costly in terms of processing.
If you are pretty sure that the resultant dataset need not have unique rows then we can skip UNION and use UNION ALL instead.
UNION ALL will be same as UNION except that it doesn't fire a DISTINCT internally sparing us costly operations