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Execution Plan: How to reduce a Sort / Distinct Sort ?

Posted on 2006-11-28
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Last Modified: 2008-02-01
Hi I have the following query:

select distinct a,b,c,d
from table
where condition1

UNION

select distinct a,b,c,d
from table
where condition2

The result returns me 6 lines from about 4000 lines of the table which is correct.
The problems is that the query is very slow, more than 4mn with a P4 Bi-processor 3Ghz !

After a look in the execution plan I notice that a Sort / distinct Sort takes me 94% of cost !

Is there a way to improve the query and to reduce the time of execution ?

Thanks & regards.


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Question by:bruno_boccara
  • 3
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7 Comments
 
LVL 35

Accepted Solution

by:
Raynard7 earned 1000 total points
ID: 18032488
One very quick thing that you could do is remove the distinct from your above queries - the union will remove duplicates anyway
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LVL 29

Assisted Solution

by:Nightman
Nightman earned 1000 total points
ID: 18032494
UNION will already apply distinct

What about this:

select a,b,c,d
from table
where condition1

UNION

select a,b,c,d
from table
where condition2

Also, consider placing a non-clustered compound covering index on columns a,b,c,d
0
 

Author Comment

by:bruno_boccara
ID: 18032534
I removed the distinct but it didn't change anything.
I still have the Sort / Distinct Sort with 94% cost.
0
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Author Comment

by:bruno_boccara
ID: 18032550
I also have indexes on columns used in condition1 and condition2
0
 
LVL 35

Expert Comment

by:Raynard7
ID: 18032665
is this from the same table or different tables? if it is from the same table can you combine the conditions into one statement?

how fast to the individual queries run? this may be the problem.
0
 

Author Comment

by:bruno_boccara
ID: 18032984
It's Ok I found the error.
there was an unwanted table in the from.

I will split the points between us to thanks you for your tries.
0
 
LVL 35

Expert Comment

by:Raynard7
ID: 18032998
glad its now working
0

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