0 - 2 of 2 tags for join optimization

I faced this scenario in an interview. Suppose there are 2 large tables say CUSTOMERS AND CUSTOMER_SESSIONS. The CUSTOMERS table holds the CUST_ID and CUST_NAME say for a website such as facebook/amazon. The CUSTOMER_SESSIONS table contains the CUST_ID referencing the CUSTOMERS tables and the SESSION_TIME every time the user logs in.

Hello, 
Consider the following situation: 
Table Alpha has the following columns:
Col1
Col2
Col3
Col4
Col5
Primary Index (Col1, Col2, Col3, Col4)
 
Table Beta has the following columns:
Col1
Col2
Col3
Col6
Col7
Primary Index (Col1, Col2, Col3)