What's the best way to create indexes (single/composite) on tables so that query will run much faster?

Let's say in below two tables,

1) table_1 has unique fields col_a, col_b, col_c and
2) table_2 has unique fields col_a, col_b,col_c, col_d

col_a, col_b, and col_c are same fields on both tables.

I will create a primary key constraint on both tables (table_1 and table_2). I will create indexes on both table for performance reason as table_1 will be joined with table_2 on col_a, col_b, col_c  

table_2 has huge number of records 400 plus millions, table_1 has less than 8 millions of records.

Questions:

1) To create a primary key constraint on both tables, should I do,
     
    create unique index idx_table_1_01 on table_1 (col_a, col_b, col_c)
    then do
    alter table table_1 add constraint pk_table_1 primary key (col_a, col_b, col_c)
   
    or just run alter table statement,
   
    alter table table_1 add constraint pk_table_1 primary key (col_a, col_b, col_c)


     
    create unique index idx_table_2_01 on table_2 (col_a, col_b, col_c, col_d)
    then do
    alter table table_2 add constraint pk_table_2 primary key (col_a, col_b, col_c, col_d)
   
    or just run alter table statement,
   
    alter table table_2 add constraint pk_table_2 primary key (col_a, col_b, col_c, col_d)

2) Since I am creating unique indexes on above two tables (table_1 and table_2). Do I still need to create normal indexes on same fields on these tables as well. For example,

create index idx_table_1_02  on table_1 (col_a, col_b, col_c)
create index idx_table_2_02 on table_2 (col_a, col_b, col_c, col_d)

or there is nop need since unique indexes on same fields exist on both tables.

3) Since I will be joining table_1 with table_2, to increase performances should I create a normal index on just a single field on both tables. For example,

create index idx_table_1_03 on table_1 (col_a)
create index idx_table_2_03 on table_2 (col_a)

or should I create composite indexes to on both tables so that query will run much faster. For example,

create index idx_table_1_03 on table_1 (col_a, col_b, col_c)
create index idx_table_2_03 on table_2 (col_a, col_b, col_c)

Just a note, col_a, col_b, and col_c are same on both tables and all three columns will be used in the WHERE clause.

Please advise.


create table table_1
(
col_a   varchar2(10 byte),
col_b   varchar2(12 byte),
col_c   varchar2(50 byte),
col_d   varchar2(10 byte),
col_i    number,
col_n   date
)

create table table_2
(
col_a   varchar2(10 byte),
col_b   varchar2(12 byte),
col_c   varchar2(50 byte),
col_d   varchar2(10 byte),
col_e   varchar2(25 byte),
col_f    number,
col_g   number,
col_s   date
)