The Psalm is based on the two decrees in vs. 1 and vs. 4.

Every word of vs. 1 will prove important and two different words for Lord are used.

Notice the use of "right hand" in vs. 1 and vs. 5.

Melchizedek only appears in the OT elsewhere in Genesis 14:18-20.
Melchizedek is king and priest. 
He blessed Abram.
Abram paid tithes to him.

The psalm gives different words to refer to the foes or enemies.
It shows us the victory over these foes is complete.
The victory is because of the LORD.