A conflicting column where NULL indicates no conflict, and other > values indicate the reason for the conflict, doesn't seem too bad. >
This is fine too.
I prefer this option. There is precedent for doing it this way, for example in pg_stat_activity.wait_event_type.
The most common test of this field is likely to be "is there a conflict" and it's better to write this as "[fieldname] IS NOT NULL" than to introduce a magic constant. Also, it makes clear to future maintainers that this field has one purpose: saying what type of conflict there is, if any. If we find ourselves wanting to record a new non-conflict status (no idea what that could be: "almost conflict"? "probably conflict soon"?) there would be less temptation to break existing tests for "is there a conflict".
