Recently I was researching several entity systems and obviously I came across T=Machine's fantastic articles on the subject. In Part 5 of the series the author uses a relational schema to explain how an entity system is built and works. Since reading this, I have been wondering whether or not actually using a compact SQL library would be fast enough for real-time usage in video games.
Performance seems to be the main issue with a full blown SQL database for management of all entities and components. However, as mentioned in T=Machine's post, basically all access to data inside the SQLDB is done sequentlially by each system over each component. Additionally, using a library like SQLite, one could easily improve performance by storing the entity data exclusively in RAM to increase access speeds.
Disregarding possible performance issues, using a SQL database, in my opinion, would allow for a very intuitive implementation of entity systems and bring a long certain other benefits like easy de/serialization of game states and consistency checks like the uniqueness of entity IDs.
Edit for clarification:
The main question was whether using a SQL database for the actual entity management (not just storing the game state on the disk) in a real-time game would still yield a framerate appropriate for a game or even if someone is aware of projects that demonstrate SQL in a video game.