Re: Identity projection

Amit Kapila <amit.kapila@huawei.com> writes:
> On Friday, February 08, 2013 12:00 AM Tom Lane wrote:
> As per my understanding, currently in code wherever Result node can be
> avoided, 
> it calls function is_projection_capable_plan(), so we can even enhance
> is_projection_capable_plan()
> so that it can also verify the expressions of tlists. But for this we need
> to change at all places 
> from where is_projection_capable_plan() is called.

Hm.  Really there's a whole dance that typically goes on, which is like

                if (!is_projection_capable_plan(result_plan))
                {
                    result_plan = (Plan *) make_result(root,
                                                       sub_tlist,
                                                       NULL,
                                                       result_plan);
                }
                else
                {
                    /*
                     * Otherwise, just replace the subplan's flat tlist with
                     * the desired tlist.
                     */
                    result_plan->targetlist = sub_tlist;
                }

Perhaps we could encapsulate this whole sequence into a function called
say assign_targetlist_to_plan(), which would have the responsibility to
decide whether a Result node needs to be inserted.

			regards, tom lane