Normally with such hierarchies it doesn't neccesary need to represent the same structure unless there is a significant difference as we can filter by animaltype with a where clause. The same approach cannot be applied to rmdbs design as to oo because the paradigms are different.
> Are there any significant differences between AnimalCat and AnimalDog? Feels like there can just be an "animals" table with an enum "type" field
Otherwise, you are going to use a foreign key lookup to get your type, which will also be an integer stored in that same row.
But, if you wanted to add a new animal type, and you used a lookup table, even if you have billion of rows, you are not messing with the table with a bunch of data. That may not be the case with an enum.
It sucks to ALTER a live table in production with billions of rows being actively INSERTed.
Edit: Basically what I am looking for is a trade-off between space and performance.