1. Do you have experience running PostgreSQL or MySQL in production? Use what you know. Do you have friends or colleagues with that experience? Use what they kn

1. Do you have experience running PostgreSQL or MySQL in production? Use what you know. Do you have friends or colleagues with that experience? Use what they know. Is the stack you end up using predominantly used with one or the other, so you have lots more people in the community who will have hit issues you might have? Use that one. Failing any of those? Default to PostgreSQL. 2. I think you mean SQL+JSON, and the answer is PostgreSQL's JSONB columns, or dumping blobs into rows in MySQL (just keep your row sizes below 4kb). 3. Possible? Yes. A good idea? Almost never. Trees and relations are quite mathematically different, and you will need to specify quite a bit of the mapping to make it come out in a way you would actually want to work with. 4. Not really...generally you would have a SQLite database. If you need stuff that doesn't go in well as a blob, you would put it in a file and include a reference in the database.