Why SELECT ... UNION is so SLOW ?
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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