GENERATE AS

Hi, I have this issue and now I'm just wasting time. Can you tell me what I'm doing wrong?

I'd like to subtract a column timestamp hour from current hour to give me hours elapased.

How do I do this easily?

I think I've tried every combination of types and casting.

Thanks in advance.
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1. ALTER TABLE matt
add column matt_time timestamp with time zone default current_timestamp;

2. select matt_time FROM matt;

2023-06-22 14:31:16.548622-04 timestamp with time zone

3. Select (date_part('hour', current_timestamp)::INT - date_part('hour', matt_time)::INT) FROM matt

0 (same hour, so 0 is OK) INT

4. ALTER TABLE auto_auth.matt
ADD column matt_hour INT GENERATED ALWAYS AS (date_part('hour', current_timestamp)::INT - date_part('hour', matt_time)::INT) stored

ERROR: generation expression is not immutable
SQL state: 42P17