Oracle slow query - sql

Question

When I query my table just like this select * from mytable, sometimes (I query the table in PLSQL developer or SQL navigator) query returns results fast and sometimes it takes 25-26 seconds. Of course, this doesn't affect the performance of business transactions.
I traced both status and it gave below results:
Fast Time:
select *
from
mytable
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 1 0.64 1.14 0 169184 0 100
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 3 0.64 1.14 0 169184 0 100
Misses in library cache during parse: 0
Optimizer mode: ALL_ROWS
Parsing user id: SYS
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 2 0.00 0.00
SQL*Net more data to client 40 0.00 0.00
SQL*Net message from client 2 0.00 0.00
********************************************************************************
Slow Time:
select *
from
mytable
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 0.00 0 0 0 0
Execute 1 0.00 0.00 0 0 0 0
Fetch 1 2.91 23.74 169076 169184 0 100
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 3 2.91 23.74 169076 169184 0 100
Misses in library cache during parse: 0
Optimizer mode: ALL_ROWS
Parsing user id: SYS
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 2 0.00 0.00
SQL*Net more data to client 40 0.00 0.00
SQL*Net message from client 2 0.00 0.00
db file scattered read **10686** 0.29 20.20
db file sequential read 6 0.00 0.01
latch: object queue header operation 1 0.00 0.00
********************************************************************************

Answer 1

At the first time, it finds all the rows in the buffer cache(see query section), memory IO is faster than disk IO.
query
----------
0
0
169076
-------
QUERY
Total number of buffers retrieved in consistent mode for all parse, execute, or fetch calls. Usually, buffers are retrieved in consistent mode for queries
The second time, the required rows are no longer available, might have flushed due to aging or space required by some other queries, hence the Oracle process has to pull all the rows from disk(see under disk section) which is slower than memory IO. And of course, the second time the query has spent most of the time on db file scattered read due to missing index on the table referenced in the query.
disk
----------
0
0
169076
-------
DISK
Total number of data blocks physically read from the datafiles on disk for all parse, execute, or fetch calls