Function ExecInitWholeRowVar

src/backend/executor/execExpr.c:2866–2935

* Prepare step for the evaluation of a whole-row variable. * The caller still has to push the step. */

 * The caller still has to push the step.
 */
static void
ExecInitWholeRowVar(ExprEvalStep *scratch, Var *variable, ExprState *state)
{
	PlanState  *parent = state->parent;

	/* fill in all but the target */
	scratch->opcode = EEOP_WHOLEROW;
	scratch->d.wholerow.var = variable;
	scratch->d.wholerow.first = true;
	scratch->d.wholerow.slow = false;
	scratch->d.wholerow.tupdesc = NULL; /* filled at runtime */
	scratch->d.wholerow.junkFilter = NULL;

	/*
	 * If the input tuple came from a subquery, it might contain "resjunk"
	 * columns (such as GROUP BY or ORDER BY columns), which we don't want to
	 * keep in the whole-row result.  We can get rid of such columns by
	 * passing the tuple through a JunkFilter --- but to make one, we have to
	 * lay our hands on the subquery's targetlist.  Fortunately, there are not
	 * very many cases where this can happen, and we can identify all of them
	 * by examining our parent PlanState.  We assume this is not an issue in
	 * standalone expressions that don't have parent plans.  (Whole-row Vars
	 * can occur in such expressions, but they will always be referencing
	 * table rows.)
	 */
	if (parent)
	{
		PlanState  *subplan = NULL;

		switch (nodeTag(parent))
		{
			case T_SubqueryScanState:
				subplan = ((SubqueryScanState *) parent)->subplan;
				break;
			case T_CteScanState:
				subplan = ((CteScanState *) parent)->cteplanstate;
				break;
			default:
				break;
		}

		if (subplan)
		{
			bool		junk_filter_needed = false;
			ListCell   *tlist;

			/* Detect whether subplan tlist actually has any junk columns */
			foreach(tlist, subplan->plan->targetlist)
			{
				TargetEntry *tle = (TargetEntry *) lfirst(tlist);

				if (tle->resjunk)
				{
					junk_filter_needed = true;
					break;
				}
			}