Why is my SELECT DISTINCT faster than a normal SELECT?

• SpitFireXVII

  SSC Veteran

  Points: 212

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  March 10, 2011 at 6:08 am

  #233822

  Hi

  I have 2 queries selecting from the same view. Both queries return the same amount of rows (317145).

  The only difference between the 2 queries is that the first one uses DISTINCT.

  So : (Query 1)

  SELECT DISTINCT col1,col2,col3

  FROM vw_myview

  ORDER BY col1,col2,col3

  (Query 2)

  SELECT col1,col2,col3

  FROM vw_myview

  ORDER BY col1,col2,col3

  When I run thr queries together with an execution plan, query 1's cost relative to the batch is 24% while query 2's cost relative to the batch is 76%. The execution plans are completely different. Query 1 runs in 5 seconds, Query 2 runs in 15 seconds.

  My question is why is the DISTINCT query 1 so much faster than query 2 when they are returning the same results?

  Thanks

  Mike

• Johan Bijnens

  SSC Guru

  Points: 135353

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  March 10, 2011 at 6:13 am

  #1297031

  You'll need to post both execution plans to be able to give you a hint on your Q.

  Johan
  
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