Re: BUG #17409: Unable to alter data type of clustered column which is referenced by foreign key

On Wed, 16 Feb 2022 at 22:38, PG Bug reporting form <noreply@postgresql.org> wrote:
> The following bug has been logged on the website:
>
> Bug reference:      17409
> Logged by:          Holly Roberts
> Email address:      holly.roberts@starlingbank.com
> PostgreSQL version: 14.2
> Operating system:   Debian 10.2.1-6
> Description:        
>
> When attempting to change the data type of a column that has previously been
> clustered on, which is also referenced by a foreign key, then an exception
> is thrown.
>
> Reproduction steps using a fresh database:
>     CREATE TABLE parent (
>         parent_field INTEGER CONSTRAINT pk_parent PRIMARY KEY
>     );
>     CREATE TABLE child (
>         child_field INTEGER,
>         CONSTRAINT fk_child FOREIGN KEY (child_field) REFERENCES parent
> (parent_field)
>     );
>     CLUSTER parent USING pk_parent;
>     ALTER TABLE parent ALTER COLUMN parent_field SET DATA TYPE BIGINT;
>
> This throws the following error:
>     ERROR:  relation 16458 has multiple clustered indexes
>     'SELECT 16458::regclass' returns 'parent';
> This has previously worked on various versions of postgres 12 and 13 for me
> (latest tried 13.6)
>

It seems the following commit cause this problem.

commit 8b069ef5dca97cd737a5fd64c420df3cd61ec1c9
Author: Peter Eisentraut <peter@eisentraut.org>

    Change get_constraint_index() to use pg_constraint.conindid

    It was still using a scan of pg_depend instead of using the conindid
    column that has been added since.

    Since it is now just a catalog lookup wrapper and not related to
    pg_depend, move from pg_depend.c to lsyscache.c.

    Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
    Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
    Reviewed-by: Michael Paquier <michael@paquier.xyz>
    Discussion: https://www.postgresql.org/message-id/flat/4688d55c-9a2e-9a5a-d166-5f24fe0bf8db%40enterprisedb.com


After some analyze, I found `ALTER TABLE parent ALTER COLUMN parent_field SET DATA TYPE BIGINT`
will split into `ALTER TABLE parent ALTER COLUMN parent_field SET DATA TYPE BIGINT` and
`ALTER TABLE public.child ADD CONSTRAINT fk_child FOREIGN KEY (child_field) REFERENCES parent(parent_field)`
statements.

When the second stement executed in RememberConstraintForRebuilding(), the
get_constraint_index() returns valid oid after 8b069ef5, however, before this
commit, it returns invalid oid.

The different is that the get_constraint_index() uses pg_depend to find
constraint index oid before 8b069ef5, after this commit it uses lsyscache
to find index oid.

I'm not sure this is a bug or not. Any thoughts?

Also Cc to Peter Eisentraut who commits this.

-- 
Regrads,
Japin Li.
ChengDu WenWu Information Technology Co.,Ltd.