DBA - Flush cache

In Oracle 10g and beyond, this command will flush the buffer cache:

alter system flush buffer_cache;

Note: Flushing the data buffer cache imposes a serious performance overhead, especially on RAC databases. Using the flush buffer cache was intended only for test system.
The following example shows the effect of flush buffer_cache (run this before and after the "alter system flush buffer_cache" statement):

select o.object_type
,      substr(o.object_name, 1, 10) as objname
,      b.objd
,      b.status
,      count(b.objd) as n
from v$bh        b
,    dba_objects o
where b.objd = o.data_object_id
and   o.owner = 'NYUSER'
group by o.object_type
,        o.object_name
,        b.objd
,        b.status
order by o.object_type
,        o.object_name

When testing the SQL it is a good idea to periodically flush the shared pool to remove older execution plans:

alter system flush shared_pool;

Note: "Flushing the shared pool should do it, but Tom Kyte lists a couple reasons below why you may not get the result you are expecting in some cases.": https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:7413988573867 and https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:6349391411093

Sources:
http://www.dba-oracle.com/t_flush_buffer_cache.htm
http://stackoverflow.com/questions/2147456/how-to-clear-all-cached-items-in-oracle