David here appeals to God, 1. As his witness that he had not done wrong; he could truly say, "I have done judgment and justice, that is, I have made conscience of rendering to all their due, and have not by force or fraud hindered any of their right." Take him as a king, he executed judgment and justice to all his people, Sa2 8:15. Take him in a private capacity, he could appeal to Saul himself that there was no evil or transgression in his hand, Sa1 24:11. Note, Honesty is the best policy and will be our rejoicing in the day of evil. 2. As his Judge, that he might not be wronged. Having done justice for others that were oppressed, he begs that God would do him justice and avenge him of his adversaries: "Be surety for thy servant, for good; undertake for me against those that would run me down and ruin me." He is sensible that he cannot make his part good himself, and therefore begs that God would appear for him. Christ is our surety with God; and, if he be so, Providence shall be our surety against all the world. Who or what shall harm us if God's power and goodness be engaged for our protection and rescue? He does not prescribe to God what he should do for him; only let it be for good, in such way and manner as Infinite Wisdom sees best; "only let me not be left to my oppressors." Though David had done judgment and justice, yet he had many enemies; but, having God for his friend, he hoped they should not have their will against him; and in that hope he prayed again, Let not the proud oppress me. David, one of the best of men, was oppressed by the proud, whom God beholds afar off; the condition therefore of the persecuted is better than that of the persecutors, and will appear so at last.

I am thy servant,.... Not only by creation, but by grace; and as he had a work to do, he desires to know what it was; and as it was proper he should know his Master's will, he applies to him for it; using this as an argument, that he was his servant, devoted to his service, and willing to perform it to the best of his knowledge and ability; and therefore prays,

give me understanding, that I may know thy testimonies; the Scriptures, which testify of the will of God; which are only rightly understood by those who have their understandings opened and enlightened; or have an understanding given them, that they may understand them, so as to receive and embrace the doctrine, and do the precepts of them: and such an understanding is the gift of God, and owing to his powerful and efficacious grace; see Luk 24:45.