Hi,
A fetch operation is supposed to return rows according to the where clause of the select a statement. The machanism is designed such a way that the rows are searched in the buffer area first, to reduce the disk I/O; which improves the performance.
In reality, it might happen that certain portions of rows are fetched from the buffer area and the balance rows are gathered from the non-buffered area (disk), according to the (non)availability cached information.
The bottom line is: you get result according to the where clause, that actually matches the data stored in database tables. Buffering or caching improves this operation.
Regards
Shriyan