translate SQL query to RAexp
for enough transformations RA' of RAexp {
   while (more choices for RelOps) {
      Plan = {};  i = 0;  cost = 0
      for each node e of RA' (recursively) {
         ROp = select RelOp method for e
         Plan = Plan ∪ ROp
         cost += Cost(ROp) // using child info
      }
      if (cost < MinCost)
         { MinCost = cost;  BestPlan = Plan }
   }
}