Doubt in index subplan query

Hello,

I have a query plan for a certain query

 Nested Loop  (cost=100000000.00..38761761090.50 rows=3000608 width=8)
   ->  Seq Scan on lineitem  (cost=100000000.00..100213649.15 rows=6001215 width=8)
   ->  Index Scan using oindex2 on myorders  (cost=0.00..6442.27 rows=1 width=4)
         Index Cond: ("outer".l_orderkey = myorders.o_orderkey)
         Filter: (subplan)
         SubPlan
           ->  Index Scan using cnation on customer  (cost=0.00..12859.39 rows=5251 width=0)
                 Index Cond: (c_nationkey = 10)

How is the subplan handled by postgres at index level ? Is any sort of hashing done ?

Thanks and regards,
Suresh