It is used when all of the columns necessary to satisfy thestatement are in the index and it is cheaper than scanning the table. It uses single block IOs.
ranit B wrote:Further to the comments made by rp0428, there is another very strong clue in the documentation that you quoted:
But just check difference between what Damorgan wrote & what you pasted from docs.
Damorgan says -
Read the entire index ... every block.
Just as a full table scan is read every block in the table.
From docs -
Full Index scan - A full index scan does not read every block in the index structure, contrary to what
its name suggests.