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

On Tue, Sep 26, 2017 at 4:57 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> =?UTF-8?Q?Fabr=C3=ADzio_de_Royes_Mello?= <fabriziomello(at)gmail(dot)com> writes:
> > I didn't came with better solution, but for now what I did is inside
> > transformaAlterTableStmt when calling transformColumnDefinition now we
pass
> > down "AlterTableStmt->missing_ok" to check and skip CREATE SEQUENCE
> > statements when use SERIAL pseudo-types.
>
> > It's not an elegant solution because during ATExecAddColumn we check it
> > again by calling check_for_column_name_collision... Ideas are very
welcome?
>
> I do not think this is a solution at all. It doesn't address the
> fundamental problem that we decide whether to make a serial sequence
> before determining whether we're going to make a column default
> depending on it.

I tried to address only the ALTER TABLE ... ADD COLUMN IF NOT EXISTS
statement, and do not touch CREATE TABLE statements...

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;

And what I tried to implement is skip step 1 and 3... and fo step 2 skip
the DEFAULT constraint

> What it does do is introduce a different set of failure
> conditions, basically race conditions around the discrepancy between
> parse-time and execution-time state.
>

I didn't understand what you mean here...

> 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... :-(

> We could maybe fix things by arranging to create the sequence only after
> we've made the column successfully, but that would be messy.

If we do that we should add more steps to execution queue...

> 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...

Regards,

--
Fabrízio de Royes Mello
Consultoria/Coaching PostgreSQL
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