1 David the king was one man, but not one man he figured; sometimes to wit he figured the Church of many men consisting, extended even unto the ends of the earth: but sometimes One Man he figured, Him he figured that is Mediator of God and men, the Man Christ Jesus. In this Psalm therefore, or rather in this Psalm's title, certain victorious actions of David are spoken of:...“To the end, in behalf of those men that shall be changed unto the title's inscription, unto teaching for David himself, when he burned up Mesopotamia in Syria, and Syria Sobal, and turned Joab, and smote Edom, in the valley of salt-pits twelve thousand.”
We read of these things in the books of the Reigns, that all those persons whom he has named, were defeated by David, that is, Mesopotamia in Syria, and Syria Sobal, Joab, Edom. These things were done, and just as they were done, so there they have been written, so they are read: let him read that will. Nevertheless, as the Prophetic Spirit in the Psalms' titles is wont to depart somewhat from the expression of things done, and to say something which in history is not found, and hence rather to admonish us that titles of this kind have been written not that we may know things done, but that things future may be prefigured....But here this thing is inserted for this special reason, that there it is not written that he burned up Mesopotamia in Syria, and Syria Sobal.
But now let us begin to examine these things after the significations of things future, and to bring out the dimness of shadows into the light of the word.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)