Indexing foreign keys

Hello all,

Because we ran into some locking problems, we're now indexing our foreign keys that have no index yet. Because it's an existing database which has been around for some time, there are a lot of foreign keys of which the column appears in an index. I was wondering if I have to put a separate index on those foreign key columns anyway, or that the locking problem will be prevented by the existing FK.

As an example:

Table TAB_PARENT has column A as primary key

Table TAB_CHILD has columns A, B, C, D, E, F  On this table, an index exists which references D,E,F, A, in that order. There's a FK on column TAB_CHILD.A, referencing TAB_PARENT.A