Jamieson, Fausset and Brown on Psalms 110:4

4. The perpetuity of the priesthood, here asserted on God's oath, corresponds with that of the kingly office just explained.

after the order--(He 7:15) after the similitude of Melchisedek, is fully expounded by Paul, to denote not only perpetuity, appointment of God, and a royal priesthood, but also the absence of priestly descent and succession, and superiority to the Aaronic order.