How ClickHouse Runs UNION, INTERSECT and EXCEPT
UNION ALL merges streams, UNION DISTINCT adds a hash DISTINCT with a self-abandoning preliminary pass, and INTERSECT/EXCEPT filter the left side through a set of the right.
UNION ALL merges streams, UNION DISTINCT adds a hash DISTINCT with a self-abandoning preliminary pass, and INTERSECT/EXCEPT filter the left side through a set of the right.