I think if you really wanted to get comfortable with something that could have you make a performant, database quickly, you might want to start with something like C...

Now, C is a shotgun that you will invariably use to shoot yourself in the foot with, but I think it's absolutely straightforward and powerful manual memory management, as well as raw speed, would make it worth learning.

Like, you could for example just focus on building a database of a given size, in memory -- you could even do it all in RAM, if you wanted.

You could have the database specialize in holding one kind of data structure (for example, how about tries? they're pretty rare, despite how useful they are)