Tehillim. Psalm 110. Who is the King?

This is a chapter spoken in God's name, seating an unnamed king on a chair at his right-hand side and promising victory over his enemies, priesthood to the nations, and the role of dispensing justice to the world. Who is the king? Abraham? King David? Mashiach? We speak about the Radak's interpretation, refuting the Christian reading of the chapter which identified the king as Jesus, and we ask why Radak needed to expend such extensive efforts if his audience was a Jewish one.