Midrasch zu Divrej Hajamim II 17:6

(Deut. 16:18:) “[You shall appoint] judges and law officers,” “judges” are magistrates, and “law officers” are administrators that supervise the people.¹¹Cf. Sifre to Deut. 16:18 (144). R. Eleazar said, “If there is no law officer, there is no judge. How so? When someone is found by a court to have a legal obligation to his companion, if there is no law officer who will collect from him when he withdraws from the court, there is no power in the hands of the judge to do anything to him. If, however, [a law officer is present], he delivers him into the hand of the law officer, and the law officer extracts compliance from him.” R. Eleazar ben Pedat said, “If it had not been for the law-enforcing office¹²Siteno, which is being read as sitero, as suggested by Jastrow, s.v. of Joab, David could not have enforced justice.” And so it says (in I Chron. 18:14-15 // II Sam. 8:15-16), “So David administered judgment and righteousness to all his people. And Joab ben Zeruiah was over the army.” Were David and Joab judges together? It is simply that whenever someone did not heed the judge, they delivered him into the hands of Joab, and he extracted [compliance] from him against his will. And so Job said (in Job 29:16-17), “I was a father to the poor […]. I broke the jaws [of the evildoer].”