I think even having a working knowledge of common algorithms and data-structures is insufficient preparation for these. They often call for incredibly esoteric algos/datastructures or test for patterns incredibly uncommon in industry, i.e dynamic programming.

I would say even if you had a good background in how to implement a proper production ready database, i.e b+trees, bloom filters, journaling, etc. This while certainly enough to do the job of building a database would be insufficient to pass the interview to build a database.

Which to me seems like the system is broken.