General subselect question

[Tim Hart <tjhart@xxxxxx>]

In the general case, is a subselect that uses union less performant  
than a union?

I have a query that looks something like this:

select <some columns>
from   table1,
          table2,
          table3
where <where clause>
union
select <select clause>
from    table1,
           table2,
           table3 ref1,
           table3 ref2
where <where clause>

I realized today that I could convert the query to

select <some columns>
from    table1,
           table2,
           ( select <some columns>
              from   table3
              where <where clause>
              union
              select <some columns>
              from    table3 ref1,
                         table3 ref2
              where  <where clause> )
where    <where clause>

For my specific case, I'll use EXPLAIN   to determine the performance  
difference. I'll weigh the performance benefits with the maintenance  
benefits ( less duplication of code for me on the second case) and  
decide (assuming I'm still motivated on Friday ;) ).

But in the general case, do I have to pay special attention with  
unions in subselects?

Tim Hart
615-713-9956 :cell
timothyjhart :Y!
tjhart@xxxxxx :AIM

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