In early 2001, for the project I was working on then, we needed something like a SQLite-lite. (SQLite existed, but was less than a year old and was still way bigger than our needs.) One of the other engineers and I paired to build something that worked well enough to run with over the course of a weekend, and then polished and improved it along the way, until the project was abandoned a few months later, because of the dot-com bomb.
I did this alone. The database itself was only about 200 lines of code but took about three weeks of effort which included writing the initial implementation, refactoring it, writing performance test, optimizing the implementation for the intended load and integrating in the application.
The application was much larger, about 40k LOC and took about two and a half years to complete, certify and deploy to production.
As I said, there is nothing spectacular about this.
This notion that a database must be a difficult and complex thing and best left for experts is just turning people off from exploring and learning.
When it doesn't need to be general purpose, has only one thread accessing it, stores at most 1MB of data, has only KV access (so basically persistent hashmap) and doesn't have to be blazing fast, the resulting code may be very simple.
