> A more "Mongo" approach is to migrate data as you need to; e.g. you pull in an older document and at that time add any missing fields.
I think this works when you're talking about adding fields. But it really breaks down if you need to make some change regarding "sub-objects" or arrays of "sub-objects".
If you have made a modeling mistake and you need to pull out a sub-object you generally have to do simply stop the system and migrate.
Which means it is not too much unlike a quality RDBMS like PostgreSQL. For adding (remember to not set a default value for large tables) and removing fields in PostgreSQL there is no requirement for locking the tables more than an instant. But for complicated schema changes you may have to either force a long lock or use some messy plan either involving replication or doing the migration in multiple steps.
I think this works when you're talking about adding fields. But it really breaks down if you need to make some change regarding "sub-objects" or arrays of "sub-objects".
If you have made a modeling mistake and you need to pull out a sub-object you generally have to do simply stop the system and migrate.