What database to choose to store the tree structure?

Question

Database

For a small project, you need a lightweight database that allows you to store a tree structure. For example, writing in json does not make sense, it is not safe, since there are no "locks", the data in the database will be updated frequently. I completely refuse SQL.

Key : value
key : value
Key :
       Key : value
       Key : value

Answer 1

MongoDB
JSONB+PostgreSQL
The classical implementation of the hierarchy for SQL is inefficient, but the same is possible. Any SQL.
A dedicated hierarchical DBMS is best, but again, what's the point if you have a small load.

Answer 2

PostgreSQL + JSONB