The main thing to learn about btrees is that they have very high branching to ensure shallowness and thus trade off more sequential accesses for fewer random accesses, a good tradeoff for disk I/O. The other thing is to recognise that sorted access is still better than random access even over an index; better for disk / block cache, better for CPU cache, and seemingly redundant sorts can e.g. have outsize constant factor performance wins for some joins over materialized subtable queries.