code: function(left, right, cost, identifier, allVars, joinVars) {
  
      this.left =  left;
      this.right = right;
      this.cost = cost;
      this.i = identifier;
      this.vars = allVars;
      this.join = joinVars;
  
  }
docs: A new query plan object
  @param left
  @param right
  @param cost
  @param identifier
  @param allVars
  @param joinVars
  @constructor