Re: BUG #14827: "ALTER TABLE... IF NOT EXISTS...ADD.. BIGSERIAL" leaves extra sequences

On Tue, Sep 26, 2017 at 1:42 PM, Fabrízio de Royes Mello <
fabriziomello(at)gmail(dot)com> wrote:

> On Tue, Sep 26, 2017 at 4:57 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> I tried to address only the ALTER TABLE ... ADD COLUMN IF NOT EXISTS
> statement, and do not touch CREATE TABLE statements...
>

​And since our docs don't explain the equivalence in terms of ALTER TABLE
we are not falsifying anything.​

For example when we add a new SERIAL column to a relation:
>
> ALTER TABLE foo ADD COLUMN bar SERIAL;
>
> What I understood is actually PostgreSQL will convert it to:
>
> 1. CREATE SEQUENCE foo_bar_seq;
> 2. ALTER TABLE foo ADD COLUMN bar INTEGER DEFAULT nextval('foo_bar_seq');
> 3. ALTER SEQUENCE foo_bar_seq OWNER BY foo.bar;
>
> ​[...]​

> > I don't feel like this is exactly a "must fix" problem, and it certainly
> > isn't one that we should fix by introducing different oddities of
> > behavior.
> >
>
> When I see the code I felt the same... :-(
>

​Agreed, but it seems worthwhile to consider making it work as the OP
expected.​

> > I'm also
> > not clear on how to document it. The documentation right now is quite
> > clear, and accurate, about what SERIAL does:
> > https://www.postgresql.org/docs/devel/static/datatype-
> numeric.html#datatype-serial
> > This patch falsifies that, and so would any other conditional creation
> > of the sequence.
> >
>
> This patch doesn't falsifies that, because will act just when IF NOT
> EXISTS is used...
>
>
​And we already deviate for ALTER TABLE by not strictly adhering to the
specified format: tablename_colname_seq; instead we allow for appending "N"
to the end of the name if necessary to make the sequence name unique.

It seems like we'd want to invoke:

CREATE SEQUENCE IF NOT EXISTS tablename_colname_seq

If the corresponding add column is likewise IF NOT EXISTS.

If we detect the column was newly created maybe then also issue a RESET
SEQUENCE just in case we picked up a left-over? This feels both cleaner
and more dangerous than just inspecting everything first and deciding how
to proceed on both fronts.

David J.