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Message Posted: Sun, 06 Dec 2004 @ 00:58:14 GMT

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I agree 100% with Dieter regarding faster and more efficient performance of GROUP BY (with lots of duplicate values) versus DISTINCT (with few duplicates).

When you look at an EXPLAIN for a subquery you will notice that it no longer eliminates duplicates. In the old days it did a "sort" and "eliminate duplicate values" in the EXPLAIN. For a couple of releases now it eliminates duplicates without eliminating duplicates - how is that for efficiency. In reality what it does is an inclusion merge join instead of a merge join. The difference is that the inclusion merge stops looking for matches on a value in the subquery rows when it finds one that matches - hence no duplicates found. Pretty clever little cleaver.

Regards,

Michael Larkins
Certified Teradata Master
Certified Teradata SQL Instructor

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