All tables had this; so model it in a separate column. Sadly Postgres' table inheritance doesn't inherit constraints, so we have to unroll that constraint for all the subtables. You will need to dump/load databases after this change (and most schema changes actually!)
Columns can have NULL values.
This is the version used for exporting the first 81 commits of the p4 repo, and will suffice until later when attribution information must be extracted from the commit message.