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Message Posted: Fri, 25 Feb 2005 @ 08:33:57 GMT

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Narayan Murthy wrote:

If you rewrite intersect/union with and/or it's usually more efficient:

- two accesses to the base table vs. one access

- distinct vs. no distinct

I usually recommend union/intersect/except only if the data is from different tables.

On some rare cases when set operations may be more efficient, but you have to EXPLAIN...

There's only one case when union is always better, a OR-ed join condition:

     select
     from t1 join t2
     on t1.col1 = t2.col1
     OR t1.col1 = t2.col2

This will *always* result in a product join, so rewrite as

     select
     from t1 join t2
     on t1.col1 = t2.col1
     union [all]
     select
     from t1 join t2
     on t1.col1 = t2.col2


Dieter

 
 
 
 


 
 
 
 
 
 
 
 
 
 
 
 
 
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