From the additional notes to Charles Spurgeon's The Treasury of David, Psalm 109.
Verse 31. He shall stand at the right hand of the poor. This expression implies, first, that he appears there as a friend. How cheering, how comforting it is to have a friend to stand by us when we are in trouble! Such a friend is Jesus. In the hour of necessity he comes as a friend to stand by the right hand of the poor creature whose soul is condemned by guilt and accusation. But he stands in a far higher relation than that of a friend; he stands, too, as surety and a deliverer. He goes, as it were, into the court; and when the prisoner stands at the bar, he comes forward and stands at his right hand as his surety and bondsman; he brings out of his bosom the acquittance of the debt, signed and sealed with his own blood, he produces it to the eyes of the court, and claims and demands the acquittal and absolution of the prisoner at whose right hand he stands. He stands there, then, that the prisoner may be freely pardoned, and completely justified from those accusations that condemn his soul. O sweet standing! O blessed appearance!—Joseph C. Philpot (1802-1869).*
Verse 31. He shall stand at the right hand of the poor. One of the oldest Rabbinical commentaries has a very beautiful gloss on this passage. "Whenever a poor man stands at thy door, the Holy One, blessed be His Name, stands at his right hand. If thou givest him alms, know that thou shalt receive a reward from Him who standeth at his right hand."—Alfred Edersheim, in "Sketches of the Jewish Social Life in the Days of Christ," 1876.
* The link to Philpot is in Italian, but Google will happily translate it into a reasonable English for you.