performance of union versus union all

Question

I have to run a select statement across several tables. I am sure the tables return different records. I am anyway using UNION ALL.

Is it better to use UNION or of U

Answer 1

Why is UNION ALL faster? Because UNION must do a sort to remove the duplicates. If you do not need to remove duplicates then UNION ALL is the better option, however UNION does have a purpose and should be used when appropriate.