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

On Sat, Apr 13, 2024 at 9:13 PM jian he  wrote:
>
> > > > Here is a patch implementing this. It was a bit more fuss than I expected, so maybe someone has a
> > > > better way.
> I think I found a simple way.
>
> the logic is:
> * ATExecAlterColumnType changes one column once at a time.
> * one column can only have one collation. so we don't need  to store a
> list of collation oid.
> * ATExecAlterColumnType we can get the new collation (targetcollid)
> and original collation info.
> * RememberAllDependentForRebuilding  will check the column dependent,
> whether this column is referenced by a foreign key or not information
> is recorded.
> so AlteredTableInfo->changedConstraintOids have the primary key and
> foreign key oids.
> * ATRewriteCatalogs will call ATPostAlterTypeCleanup (see the comments
> in ATRewriteCatalogs)
> * for tab->changedConstraintOids (foreign key, primary key) will call
> ATPostAlterTypeParse, so
> for foreign key (con->contype == CONSTR_FOREIGN)  will call TryReuseForeignKey.
> * 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.
> so foreign key can act accordingly at ATAddForeignKeyConstraint (old_check_ok).
>
>
> based on the above logic, I add one bool in struct AlteredTableInfo,
> one bool in struct Constraint.
> bool in AlteredTableInfo is for storing it, later passing it to struct
> Constraint.
> we need bool in Constraint because ATAddForeignKeyConstraint needs it.

I refactored the comments.
also added some extra tests hoping to make it more bullet proof, maybe
it's redundant.