29 Lastly, he adds, “The tongue of Your dogs out of enemies by Himself,” calling those very same that had been about to strive for the faith of the Gospel, even dogs, as though barking for their Lord. Not those dogs, whereof says the Apostle, “Beware of dogs:” but those that eat of the crumbs which fall from the table of their masters. For having confessed this, the woman of Canaan merited to hear, “O woman, great is your faith, be it done to you as you will.” Dogs commendable, not abominable; observing fidelity towards their master, and before his house barking against enemies.
Not only “of dogs” he has said, but “of Your dogs:” nor are their teeth praised, but their tongue is: for it was not indeed to no purpose, not without a great mystery, that Gedeon was bidden to lead those alone, who should lap the water of the river like dogs; and of such sort not more than three hundred among so great a multitude were found. In which number is the sign of the Cross because of the letter T, which in the Greek numeral characters signifies three hundred. Of such dogs in another Psalm also said, “They shall be turned at even, and hunger they shall suffer as dogs.” For even some dogs have been reproved by the Prophet Isaiah, not because they were dogs, but because they knew not how to bark, and loved to sleep. In which place indeed he has shown, that if they had watched and barked for their Lord, they would have been praiseworthy dogs: just as they are praised, of whom is said, “The tongue of Your dogs.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)