---会话1创建测试表并插入数据
SQL> create table t_network(a int);

Table created.

--构建索引,产生db file sequential read
SQL> create index idx_t_single on t_single(a);

Index created.

SQL> insert into t_network values(1);

1 row created.

SQL> commit;

Commit complete.

--会话 清空共享池,便于观察 db file sequential read
SQL> alter system flush shared_pool;
 
System altered

SQL>  select event,total_waits,time_waited,average_wait from v$session_event  where sid=1 order by total_waits desc;
 
EVENT                                                            TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
---------------------------------------------------------------- ----------- ----------- ------------
db file sequential read                                                  139          99         0.71  --此等待事件增大，是因为会读取与本话相关的一些字典表的数据
SQL*Net message to client                                                 24           0            0
SQL*Net message from client                                               23      260608     11330.78
log file sync                                                              5           0         0.09
Disk file operations I/O                                                   4           0         0.06
events in waitclass Other                                                  3           6         1.93
SQL*Net break/reset to client                                              2           0         0.01
enq: RO - fast object reuse                                                1           1         0.81
 
8 rows selected

---读取表记录
SQL> select * from t_single;

         A
----------
         1
         2

---对比发现db file sequential read等待事件加大了
SQL> /
 
EVENT                                                            TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
---------------------------------------------------------------- ----------- ----------- ------------
db file sequential read                                                  142         104         0.73
SQL*Net message to client                                                 26           0            0
SQL*Net message from client                                               25      263763     10550.52
log file sync                                                              5           0         0.09
Disk file operations I/O                                                   4           0         0.06
events in waitclass Other                                                  3           6         1.93
SQL*Net break/reset to client                                              2           0         0.01
enq: RO - fast object reuse                                                1           1         0.81
 
8 rows selected

---表已在buffer cache中
SQL> /

         A
----------
         1
         2

---对比发现如表已在buffer cache则db file sequential read不会增加
SQL> /
 
EVENT                                                            TOTAL_WAITS TIME_WAITED AVERAGE_WAIT
---------------------------------------------------------------- ----------- ----------- ------------
db file sequential read                                                  142         104         0.73
SQL*Net message to client                                                 28           0            0
SQL*Net message from client                                               27      280384     10384.58
log file sync                                                              5           0         0.09
Disk file operations I/O                                                   4           0         0.06
events in waitclass Other                                                  3           6         1.93
SQL*Net break/reset to client                                              2           0         0.01
enq: RO - fast object reuse                                                1           1         0.81
 
8 rows selected
 
 
小结:
     1,db file sequential read即服务器进程发出读取表记录所属的数据块时，如不在buffer cache,则调用io自物理磁盘把对应的数据块读取到buffer cache，服务器进程等待这个io动作完成的过程
     2,所读取的数据块不在buffer cache而在物理磁盘上