Function MJFillOuter

src/backend/executor/nodeMergejoin.c:483–511

* Generate a fake join tuple with nulls for the inner tuple, * and return it if it passes the non-join quals. */

 * and return it if it passes the non-join quals.
 */
static TupleTableSlot *
MJFillOuter(MergeJoinState *node)
{
	ExprContext *econtext = node->js.ps.ps_ExprContext;
	ExprState  *otherqual = node->js.ps.qual;

	ResetExprContext(econtext);

	econtext->ecxt_outertuple = node->mj_OuterTupleSlot;
	econtext->ecxt_innertuple = node->mj_NullInnerTupleSlot;

	if (TupIsNull(node->mj_OuterTupleSlot))
		return NULL;

	if (ExecQual(otherqual, econtext))
	{
		/*
		 * qualification succeeded.  now form the desired projection tuple and
		 * return the slot containing it.
		 */
		MJ_printf("ExecMergeJoin: returning outer fill tuple\n");

		return ExecProject(node->js.ps.ps_ProjInfo);
	}
	else
		InstrCountFiltered2(node, 1);

	return NULL;
}

/*
 * Generate a fake join tuple with nulls for the outer tuple,