XPost: microsoft.public.sqlserver.programming   
   From: bod@newsguy.com   
      
   Doesn't UNION imply a distinct over the entire combined result set?   
   (effectively SELECT DISTINCT * FROM (SELECT ... UNION ALL SELECT ...))   
   If you can construct the query such that each half of the union is   
   exclusive, then UNION ALL may yield better performance.   
      
   On 6/23/2010 7:16 PM, Erland Sommarskog wrote:   
   > Query Builder (querybuilder@gmail.com) writes:   
   >   
   > A good start is to look at the query plans, at least the estimated   
   > plan for the UNION query. Since the query never seem to complete,   
   > looking at the actual plans may be more difficult.   
   >   
   > You say that each section of the UNION returns 200 rows, but how big   
   > are the underlying tables? That is, is there any potential for a   
   > query plan from hell?   
   >   
   >   
   >   
   >   
      
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