No, you can safely drop a foreign key referential integrity constraint without affecting any underlying indexes.
SQL> create table p(t number primary key); Table created. SQL> create table c(f number primary key, t number references p(t)); Table created. SQL> create index c_t on c(t); Index created. SQL> select index_name from user_indexes where table_name = 'C'; INDEX_NAME ------------------------------ SYS_C00413196 C_T SQL> select constraint_name from user_constraints where r_constraint_name in (select constraint_name from user_constraints where table_name = 'P'); CONSTRAINT_NAME ------------------------------ SYS_C00413197 SQL> insert into c values(1,1); insert into c values(1,1) * ERROR at line 1: ORA-02291: integrity constraint (FOO.SYS_C00413197) violated - parent key not found SQL> alter table c drop constraint SYS_C00413197; Table altered. SQL> select constraint_name from user_constraints where r_constraint_name in (select constraint_name from user_constraints where table_name = 'P'); no rows selected SQL> select index_name from user_indexes where table_name = 'C'; INDEX_NAME ------------------------------ SYS_C00413196 C_T SQL> alter table c add constraint c_t_fk foreign key(t) references p(t); Table altered. SQL> select constraint_name from user_constraints where r_constraint_name in (select constraint_name from user_constraints where table_name = 'P'); CONSTRAINT_NAME ------------------------------ C_T_FK SQL> select index_name from user_indexes where table_name = 'C'; INDEX_NAME ------------------------------ SYS_C00413196 C_T SQL>