Re: altering a column's collation leaves an invalid foreign key

On Sat, Jun 8, 2024 at 4:12 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> jian he <jian(dot)universality(at)gmail(dot)com> writes:
> >> * in TryReuseForeignKey, we can pass the information that our primary
> >> key old collation is nondeterministic
> >> and old collation != new collation to the foreign key constraint.
>
> I have a basic question about this: why are we allowing FKs to be
> based on nondeterministic collations at all? ISTM that that breaks
> the assumption that there is exactly one referenced row for any
> referencing row.
>

for FKs nondeterministic,
I think that would require the PRIMARY KEY collation to not be
indeterministic also.

for example:
CREATE COLLATION ignore_accent_case (provider = icu, deterministic =
false, locale = 'und-u-ks-level1');
DROP TABLE IF EXISTS fktable, pktable;
CREATE TABLE pktable (x text COLLATE ignore_accent_case PRIMARY KEY);
CREATE TABLE fktable (x text REFERENCES pktable on update cascade on
delete cascade);
INSERT INTO pktable VALUES ('A');
INSERT INTO fktable VALUES ('a');
INSERT INTO fktable VALUES ('A');
update pktable set x = 'Å';
table fktable;

if FK is nondeterministic, then it looks PK more like FK.
the following example, one FK row is referenced by two PK rows.

DROP TABLE IF EXISTS fktable, pktable;
CREATE TABLE pktable (x text COLLATE "C" PRIMARY KEY);
CREATE TABLE fktable (x text COLLATE ignore_accent_case REFERENCES
pktable on update cascade on delete cascade);
INSERT INTO pktable VALUES ('A'), ('Å');
INSERT INTO fktable VALUES ('A');

begin; delete from pktable where x = 'Å'; TABLE fktable; rollback;
begin; delete from pktable where x = 'A'; TABLE fktable; rollback;