Comments on: Aggregating in SQL across multiple levels and along several dimensions using ROLLUP and GROUPING SETS Friends of Oracle and Java Sat, 01 Aug 2015 11:10:05 +0000 hourly 1 By: Viliam Wed, 19 Apr 2006 07:53:37 +0000 A remark to a statement, that UNION (ALL) is “less than perfect performing”:
* UNION is bad performing, it has to sort all queries and remove duplicates
* UNION ALL is perfectly performing, it just executes first query, returns rows, executes the second and returns rows. It is equal to executing both queries.