You could just target your database at data analysis. Then you
don't need to care about ACID, transactions etc. Just load all
the data into memory, and start analyzing it.
Also, you'd typically be scanning over large parts of the data
set for each query, so you may not need to support a full query
language. Just what is needed for data analysis.
Later you can modify your engine to support ACID, more expressive
query language etc.
On one of the projects I am working on right now, we will also
implement our own database engine. We need it to integrate
tightly with the rest our architecture, and the only way to do
that is to roll our own. We will also not be using SQL because
SQL is so limiting.
So, I'd say "go ahead" - you can only learn something from the
project. I've "reinvented a lot of wheels" over the years, and
each time I came out smarter than before. Not every reinvention
was a success, but I always learned something from the process.
