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Message Posted: Sun, 02 Mar 2003 @ 14:22:16 GMT

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Subj:   Re: Performance for Distinct values
From:   David Wellman

The second point that you refer to here is irrelevant.

In an insert/select the select is treated as a sub-query. Teradata never returns duplicate rows from a sub-query (note that a derived table is not a sub-query), hence the de-duplication is done prior to the Insert phase, which means that Set/Multiset is irrelevant. When you don't code either a GROUP BY or a DISTINCT Teradata will use DISTINCT processing logic.

It is the other factors which determine whether the GROUP BY or DISTINCT processing goes faster -- i.e. the number of rows/value.


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