It seems like the closest thing to this in widespread use is a SQL database. If you want to change a table, you can run an ALTER TABLE command.
But of course you don't want to do this in the live, production database unless you're a DBA who is doing an upgrade manually for some (probably bad) reason.
So, you can try out your ALTER TABLE commands in a scratch database, but you'll need to save them to a migration script, and test the migration.
It seems like a weakness of this sort of live updating? Sure, you can modify your own running instance, but upgrading the production instance(s) will often still be a lot of work. Migrating a production database schema tends to be a tricky thing.
But of course you don't want to do this in the live, production database unless you're a DBA who is doing an upgrade manually for some (probably bad) reason.
So, you can try out your ALTER TABLE commands in a scratch database, but you'll need to save them to a migration script, and test the migration.
It seems like a weakness of this sort of live updating? Sure, you can modify your own running instance, but upgrading the production instance(s) will often still be a lot of work. Migrating a production database schema tends to be a tricky thing.