The following bug has been logged online:

Bug reference: 2073
Logged by: Aaron Dummer
Email address: aaron (AT) dummer (DOT) info
PostgreSQL version: 8.0.3
Operating system: Debian Linux
Description: Can't drop sequence when created via SERIAL column
Details:

If I create a table named foo with a column named bar, column type SERIAL,
it auto-generates a sequence named foo_bar_seq. Now if I manually create a
new sequence called custom_seq, and change the default value of foo.bar to
reference the new sequence, I still can't delete the old sequence
(foo_bar_seq).

In other words, from a user's point of view, the foo table is no longer
dependent on the foo_bar_seq, yet the system still sees it as dependent.

I brought this topic up on the #postgresql IRC channel and the behavior was
confirmed by AndrewSN, scampbell_, and mastermind.

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Aaron Dummer wrote:
Right. We have this TODO item:

* %Disallow changing default expression of a SERIAL column?

which would prevent you from changing the default expression for a
SERIAL column. So the answer is, don't do that, and in the future, we
might prevent it.

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