Great writeup !

The bitmap trick is elegant and I've seen similar patterns in other contexts. The core insight resonates beyond Rust and SQL: the data structure that's "obvious" at design time can become a bottleneck when the real-world usage pattern diverges from your assumptions. Most fields exist vs most fields might not exist is a subtil but critical distinction.

The fix being a simple layout change rather than a clever algorithm is also a good reminder. I've spent 20 years building apps and the most impactful optimizations were almost always about changing the shape of data, not adding complexity.