SQL & PL/SQL :: Performance With Materialized View Fast Refresh

I've 2 tables in schema A.

1) TXN_HD_FCT (This table has 6 billion records. Has fields TXN_DT, CUST_ID, TXN_AMT, TXN_ID and the table is partitioned on TXN_DT. We load data approximately 600K+ records to this table every day
2) RELINK_CUST (This table has 2 fieds CUST_ID and NEW_CUST_ID). We load data approximatley 100-200K+ records every day into this table. This table has total 10MM+ records now.

Now i need to create a table (TXN_UPD_HDR_FCT) in Schema B by joining above 2 tables using below query every day

CREATE TABLE TXN_UPD_HDR_FCT
select a.TXN_DT,
nvl(b.new_cust_id, a.cust_id) as cust_id,
a.txn_amt,
a.txn_id,
from schema_A.TXN_HD_FCT a,
schema_A.RELINK_CUST b
where a.cust_id = b.cust_id(+);

We update CUST_ID to NEW_CUST_ID based on some rules (and keep it in the table RELINK_CUST) and need to take this new_cust_id while building the table TXN_UPD_HDR_FCT.

Building this table is taking approximately 4+ hours every day. just refresh only records from TXN_HDR_FCT whose CUST_ID is changed or any records inserted into TXN_HDR_FCT? I read about Materialized views but not sure how can i create MV logs on 2 tables? If i use materialized views do i need to create indexes every day?