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Index Unique Scan

Notes:

The “index unique scan” returns 0 or 1 ROWID and is used when a condition includes a primary or unique key.
CUST_NO is the primary key of the CUSTOMER table. Oracle executes this query by:
Traversing the b-tree index to acquire the required ROWID from the leaf block
Pass that ROWID to the TABLE ACCESS BY ROWID step which then reads the correct table block
The CBO calculates the cost for a unique index scan as :
blevel + 1
where blevel is the number of levels in the index.