Minimalist demo for unindexed foreign key

Oracle Database Transactions and Locking Revealed by Thomas Kyte, Darl Kuhn
https://books.google.com/books?id=zV8nCgAAQBAJ&pg=PA23

Session 1:
create table p (x int primary key);
create table c (x references p);
--create index idx_c_x on c (x);
insert into p values (1);
insert into p values (2);
commit;
insert into c values (2);

Session 2:
delete from p where x=1; <-- this will hang unless idx_c_x exists

Comment: 
The index idx_c_x helps by reducing the mode of TM lock on c requested by Session 2 from 4 (which 
cannot be got and Session 2 will wait) to 3 (which can be got). The index is useful in eliminating 
blocking even in cases it has low uniqueness; in that case it won't help with performance.

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When does a foreign key not need an index?

Tom Kyte says:
https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:292016138754

So, when do you NOT need to index a foriegn key. In general when the following conditions are met:

o you do NOT delete from the parent table. (especially with delete cascade -- it is a double whammy)
o you do NOT update the parent tables unique/primary key value.
o you do NOT join from the PARENT to the CHILD (like DEPT->EMP).

If you satisfy all three above, feel free to skip the index, it is not needed. If you do any of the 
above, be aware of the consequences.