>selection</em>
     (discarding rows that do not meet a specified Boolean condition)
     and <em class="firstterm">projection</em> (computation of a derived column set
     based on given column values, that is, evaluation of scalar
     expressions where needed).  One of the responsibilities of the
     planner is to attach selection conditions from the
     <code class="literal">WHERE</code> clause and computation of required
     output expressions to the most appropriate nodes of the plan
     tree.
 