ORACLE uses two ways to access records in the table:
a. full table scan
Full table scan table is accessed sequentially each record. The ORACLE read by way of a plurality of data blocks (database block) to optimize full table scan.
b. access the table by ROWID
You can access method based ROWID cases, improve the efficiency of access to the table,, ROWID contains the physical position information recorded in the table using ..ORACLE index (INDEX) to achieve the physical location of data and storage of data (ROWID) between contact
Usually the index provides a quick way to access ROWID, so those based index column query can be improved in performance.