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This is an abstract example for the rewriting mechanism described above: 

Before rewriting:

--Op1

  --Subplan{

    --AggregateOp

      --NestedOp

        .....

          --Nested-Tuple-Source

    }

    --InputOp

      .....

After rewriting:

--Op1

  --GroupBy v_lc_1, ..., v_lc_n Decor v_l1, ....v_ln {

            --AggregateOp

              --Select v_new!=NULL

                -- Nested-Tuple-Source

          }

     --LeftOuterJoin (v_lc_1=v_rc_1 AND .... AND v_lc_n=v_rc_n)

       (left branch)

         --InputOp

            .....

       (right branch)

         -- Assign v_new=TRUE

           --NestedOp

             .....

               --Deepcopy_The_Plan_Rooted_At_InputOp_With_New_Variables(InputOp)

In the plan, v_lc_1, ..., v_lc_n are live "covering" variables at InputOp,while  while v_rc_1, ..., v_rc_n are their corresponding variables populated from the deepcopy of InputOp.( "Covering" variables form a set of variables that can imply all live variables.)v v_l1, ....v_ln in the decoration part of the added group-by operator are alllive all live variables at InputOp except the covering variables v_lc_1, ..., v_lc_n.  In the current implementation, we use "covering" variables as primary key variables. In the next version, we will use the real primary key variables.

Here is a concrete example of the general case rewriting (optimizerts/queries/nested_loj4.aql). 

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