Database kernels have some of the strictest resource behavior constraints of all software. Every one I have worked on in vaguely recent memory has managed memory. There is no dynamic allocation from the OS. Many invariants important to databases rely on strict control of resource behavior. An enormous amount of optimization is dependent on this, so performance-engineered systems generally don’t have issues with memory safety.

Modern database kernels are memory-bandwidth bound. Micro-managing the memory is a core mechanic as a consequence. It is difficult to micro-manage memory with extreme efficiency if it isn’t implicitly safe. Companies routinely run formal model checkers like TLA+ on these implementations. It isn’t a rando spaffing C++ code.

I’ve used PostgreSQL a lot but no one thinks of it as highly optimized.

This is true. But it has some weird gaps that make it difficult to express fundamental things in the low-level systems world without using a lot of “unsafe”. Or you can do it safely and sacrifice a lot of performance. I am a fan of formal verification and use it quite a lot but Rust is far more restrictive than formal verification requires.

Rust is a systems language but it is uncomfortable with core systems-y things like DMA because it breaks lifetime and ownership models, among many other well-known quirks as a systems language. Other verifiable safety models exist that don’t have these issues. C++, for better or worse, can deal with this stuff in a straightforward way.