This is a very naiive and simple question:

I really love the idea of SQL and am looking for a project to use it. It seems like a great way to encapsulate most of the state of your program so you can make your code more decoupled. For crunching logs or other data-dumps it seems like a no brainier, but for everything else I don't really have a a sense for the performance characteristics. Like would it make sense to keep objects' data in an SQL table (like a pImpl) ? is going over the rows of SQL table instead of std::vectors a different order of magnitude performance wise (I assume with something like SQL-Lite I can keep the tables in memory and not on disk).

I guess when is it appropriate to use a database and when is it not?

EDIT:

For context - I'm working almost exclusively in C++ and am usually pretty performance sensitive. In my last job we did radar simulations with lot of entities (radar platforms, aircraft, ballistic missiles etc.) that just lived in STD containers. Decoupling state from the simulation was always a bit of a struggle. Would keeping something like that in an SQL database make sense?

Another example. At the moment I'm working on an art project and I need to keep track of vectors and polygons which will be later used by a ray tracer. I'd also like to keep the state decoupled from my program. Would a SQL database be a bottleneck?