select customer_name, customer_number
from customer_dim
where source_system = 'tms_frc' (only 0.4 % of the table)

I would suggest create a normal B-tree index on SOURCE_SYSTEM. And use hint clause in the SELECT to force CBO to use the Index.

DO NOT ASSUME that CBO will always do a fine job. If you know your data and SQL, fine tune it as much as possible by adding HINTs in the SELECT statement, because it works for 1M rows as well as 10M rows.