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Why SELECT ... UNION is so SLOW ?
Posted by: Gérard LEMENN
Date: February 13, 2008 04:15AM

Hello,

I have 2 tables : T1 and T2 containing 2 columns : (a,b) , which are indexed.


I really can't figure out why :

SELECT a,b FROM T1 UNION SELECT a,b FROM T2

is much much slower than :

SELECT a,b FROM T1 GROUP BY a,b UNION SELECT a,b FROM T2 GROUP BY a,b

although both have same results. Is this a bug in the query optimizer ?

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Why SELECT ... UNION is so SLOW ?
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February 13, 2008 04:15AM


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