Justin Pryzby <pryzby@telsasoft.com> writes:
>>>> I can cause the error at will on the existing table,
That's good news, at least.
1. Please trigger it with "\set VERBOSITY verbose" enabled, so we can see
the exact source location --- there are a couple of instances of that
text.
2. Even better would be a stack trace for the call to errfinish,
https://wiki.postgresql.org/wiki/Generating_a_stack_trace_of_a_PostgreSQL_backend
3. It's pretty hard to see how you'd reach any of these places for an
ALTER COLUMN TYPE on a simple table. Has the table got rules, triggers,
default values? Could we see "\d+" output for it?
regards, tom lane