Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
create table t_parent
             ( id_parent  number constraint pk_t_parent primary key
             , val        varchar2(5)
             )
create table t_child
            ( id_child  number
            , id_parent number
            , val       varchar2(5)
            , constraint fk_t_child_t_parent foreign key (id_parent) references t_parent (id_parent)
            )
insert into t_parent( id_parent, val) values( 1, 'A')
1 rows affected

insert into t_child( id_child, id_parent, val) values( 1, 1, 'A')
1 rows affected

insert into t_child( id_child, id_parent, val) values( 2, null, 'B')
1 rows affected

alter index pk_t_parent unusable
insert into t_child( id_child, id_parent, val) values( 4, null, 'D')
ORA-01502: index 'FIDDLE_HLMXFXAVQLBTJWVNYBLL.PK_T_PARENT' or partition of such index is in unusable state

update t_child set id_parent = null where id_parent = 1
ORA-01502: index 'FIDDLE_HLMXFXAVQLBTJWVNYBLL.PK_T_PARENT' or partition of such index is in unusable state

update t_child set id_parent = null where id_parent is null
ORA-01502: index 'FIDDLE_HLMXFXAVQLBTJWVNYBLL.PK_T_PARENT' or partition of such index is in unusable state

delete from t_child where id_parent is null
ORA-01502: index 'FIDDLE_HLMXFXAVQLBTJWVNYBLL.PK_T_PARENT' or partition of such index is in unusable state

alter index pk_t_parent REBUILD
insert into t_child( id_child, id_parent, val) values( 4, null, 'D')
1 rows affected

update t_child set id_parent = null where id_parent = 1
1 rows affected

update t_child set id_parent = null where id_parent is null
3 rows affected

delete from t_child where id_parent is null
3 rows affected