block's life of Full Table Scan

When a block needs to be read into the buffer cache, Oracle must first find a free buffer in
which to store the block. The process searches the LRU list, beginning at the least recently used
end. It will search until it finds a free buffer. If it cannot find any, it will signal the DBW0 process
to flush any dirty blocks back to disk in order to make room. If no dirty blocks exist, the least
recently used block will be aged out to make room. The block will then be written to the buffer,
and the buffer moves to the most recently used end of the LRU list.

The exception to this rule is when a full table scan operation is performed. Blocks retrieved
from a full table scan are added to the least recently used end of the LRU list, so they will be aged
out quickly. This is because full table scan blocks are generally scanned quickly and are no longer
needed. This functionality can cause problems for small tables that are accessed frequently, such
as lookup tables. Small tables are meant to be accessed via full table scans. This is because a full
table scan will outperform an index lookup on a very small table.
Because Oracle 10g will put these blocks at the least recently used end of the LRU list, they
will age out quickly. As a result, the next time that the table is accessed, Oracle 10g may likely
have to perform PIO to retrieve the blocks again. In this situation, you can add the CACHE clause
to the table (either via ALTER TABLE or in the CREATE TABLE statements) to circumvent this
behavior. You can also choose to pin the table in memory.