I agree with this. B-trees vs hash indices, I/O hierarchy, process model, etc.

Also in the modern day, it's valuable to learn common strategies behind column-oriented databases such as late tuple materialization, deferred execution, linear scans vs. binary search, instruction pipelining.

Once you're familiar with all this, you'd find sometimes in the field what you really need is a simple flat file or an embeddable database like RocksDB, not a DBMS.