Database Tuning (MOSC)

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UNION ALL slow

edited Nov 9, 2011 2:00AM in Database Tuning (MOSC) 1 commentAnswered

 I have 2 complex queries, just for the simplicity 

1. SELECT * from T1 --> takes 1sec

2, SELECT * from T2 --> takes .5 sec

3,  SELECT * from T1 --> takes 3 sec
UNION ALL
SELECT * from T2 

Why is UNION ALL taking more time? even if its not doing parallel execution it should not be taking 3 secs? any idea? T1 and T2 are complex queries, I made is to simplify the question.



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