Ask HN: Flexible Schema (Airtable Clone)

Not that I am building another Airtable clone, but I'd like to understand what the best way is to give the end users ability to add new fields. The requirement is to use a relational database. So far, I have seen two approaches. 1. Using a Json column, not a big fan of this approach, as it will be difficult to do reports and searches; 2. Create specialized data abstraction layer, that allows adding, showing fields without structural changes to database schema. Any other ideas?