It depends on the purpose. If the objective is maximum scale and performance then Zig. The low-level mechanics of userspace I/O and execution scheduling in top-end database architectures strongly recommends a language comfortable expressing complex relationships in contexts where ownership and lifetimes are unavoidably ambiguous. Zig is designed to enable precise and concise control in these contexts with minimal overhead.

If performance/scale-maxxing isn't on the agenda and you are just trying to crank out features then Rust probably brings more to the table.

The best choice is quite arguably C++20 or later. It has a deep set of somewhat unique safety features among systems languages that are well-suited to this specific use case.

First, I would avoid using any low-level language if at all possible, because no matter what you pick, the maintenance and evolution costs are going to be significantly higher than for a high-level language. It's a very costly commitment, so I'd want to be sure it's worth it. But let's suppose I decided that I must use a low-level language (perhaps because worst-case behaviour is really important or I may want to run in a low-memory device or the DB was a "pure overhead" software that aims to minimise memory consumption that's needed for a co-located resource heavy application).

Then, if this were an actual product that people would depend on for a long time, the obvious choice would be C++, because of its maturity and good prospects. But say this is hypothetical or something more adventurous that allows for more risk, then I would say it entirely depends on the aesthetic preferences of the team, as neither language has some clear intrinsic advantage over the other. Personally, I would prefer Zig, because it more closely aligns with my subjective aesthetic preferences, but others may like different things and prefer Rust. It's just a matter of taste.