For many types of operations, Oracle Database uses the buffer cache to store data blocks read from disk. Oracle Database bypasses the buffer cache for particular operations, such as sorting and parallel reads.
Buffer Cache is an other name for logical oracle RAM buffer and is a part of the in System Global Area (SGA).
The “buffer cache”:
alter system flush buffer_cache
Many people think that they must flush the buffer cache before tuning, to emulate the real world.
If you tune with logical_io (consistent_gets) in mind, the physical_io will take care of themselves over time (assuming you ensure an even distribution of I/O across disks).
Flushing the buffer cache gets rid of tons of information that rightly would be there in a production environment when you run the query.