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      <a name="trace-general-structure">
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      10.15.11 Trace General Structure
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  <p>
   A trace follows the actual execution path very closely; for each
      join, there is a join preparation object, a join optimization
      object, and a join execution object. Query transformations
      (
   <code class="literal">
    IN
   </code>
   to
   <code class="literal">
    EXISTS
   </code>
   , outer join to
      inner join, and so on), simplifications (elimination of clauses),
      and equality propagation are shown in subobjects. Calls to the
      range optimizer, cost evaluations, reasons why an access path is
      chosen over another one, or why a sorting method is chosen over
      another one, are shown as well.
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