I suspect that if you read the spec carefully it would want a "triggered data change violation" error raised here. My advice is not to use a BEFORE trigger for this.
What would you recommend then. I am using Hibernate in my java application and if the sort_order column (in this example column.c) gets out of sync, then the Hibernate application fails. In hind site, the after delete trigger would be the better choice.
The trigger works fine for one row deletes, but I noticed that you can't even do multiple row deletes with the trigger. Postgres only executes one row delete.
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