Adam Clarke's Commentary on the Bible

Verse 20 (Psalms 89:20)

I have found David my servant - This is the sum of what God had said in prophetic visions to his saints or holy persons, Samuel, Nathan, and Gad; see 1 Samuel 16:1 , 1 Samuel 16:12 . Here the psalmist begins to reason with God relative to David, his posterity, and the perpetuity of his kingdom; which promises appear now to have utterly failed, as the throne had been overturned, and all the people carried into captivity. But all these things may have reference to Christ and his kingdom; for we are assured that David was a type of the Messiah.

- Adam Clarke's Commentary on the Bible