Spanish restaurant etiquette for dogs is fairly consistent regardless of region: keep the dog on a short leash, positioned beside or under the table rather than wandering between other diners, and never on a chair or the table itself. A dog that settles quietly is far more likely to be welcomed back next time, and staff genuinely do remember.
Because Spain's law now puts the burden on a venue to post a sign if it refuses pets, the practical etiquette move is to glance at the entrance rather than assume you need to ask. If there's no sign and no other obvious indication either way, a quick "¿Puedo entrar con el perro?" at the door is still a polite, appreciated habit, especially at a smaller family-run venue.
A PPP dog needs its muzzle in any public space, restaurants included, so that rule doesn't relax just because you're indoors and seated. Bringing a small travel mat or blanket for the dog to settle on is a small touch that Spanish waitstaff and fellow diners tend to appreciate, particularly on a busy terrace or in a tightly packed tapas bar.