> How would you fund the open source development of a database?

I think the best way would be something similar to the Linux Foundation. Companies in need of a certain type of database would pool resources to develop and maintain it.

PostgreSQL was a community developed database that was funded in something approaching this way as most of the developers were either students or worked for someone who paid them while they worked on it.

> TiDB is Apache licensed, that should be enough no?

It's good enough to use it with the knowledge that it isn't truly free (libre) because the CLA gives the owning company the right to change that at any time. So you can use it but don't build a business around it or make it a critical part of your infrastructure as they might pull the rug out from under you.