[cakeface]

postgres has a version of this preempts idea with default values on columns. postgres will fill in the value at query time without needing to backfill the data. postgres is not a horizontally scalable database like FDB so not a direct comparison. In practice this means the migration lock is much shorter and it becomes possible to actually have a default in large tables.

[hchaudhary]

Allowing default values for columns is definitely doable and we can also implement it in a similar way by filling it in during the query. But changing the type of a field to an incompatible type is tricky and needs more constraints and external machinery to fix the history.