模拟DB File Sequential Read

    当发生单块读时，就会出现这种等待，大部分情况下是读索引，少部分情况下是读undo。构造索引范围扫描，用v$session_event中捕捉。
session1:

C:\Documents and Settings\guogang>sqlplus test/test

SQL*Plus: Release 11.2.0.1.0 Production on 星期四 5月 23 20:10:12 2013

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

连接到:

Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options

SQL> select distinct sid from v$mystat;

       SID

----------

        18

SQL> create table test as select * from dba_objects;

SQL> create index ind_t_id on test(object_id);

SQL> alter session set events '10046 trace name context forever,level 12';

SQL> set autotrace traceonly

SQL> alter system flush buffer_cache;
SQL> select /*+index(t ind_t_id)*/ object_id from test t where object_id > 1;

执行计划

----------------------------------------------------------

Plan hash value: 3343177607

-----------------------------------------------------------------------------

| Id  | Operation        | Name     | Rows  | Bytes | Cost (%CPU)| Time     |

-----------------------------------------------------------------------------

|   0 | SELECT STATEMENT |          | 63057 |   800K|   173   (0)| 00:00:03 |

|*  1 |  INDEX RANGE SCAN| IND_T_ID | 63057 |   800K|   173   (0)| 00:00:03 |

-----------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

   1 - access("OBJECT_ID">1)

统计信息

----------------------------------------------------------

         32  recursive calls

          0  db block gets

       5006  consistent gets

        517  physical reads

          0  redo size

    1043091  bytes sent via SQL*Net to client

      53072  bytes received via SQL*Net from client

       4789  SQL*Net roundtrips to/from client

          0  sorts (memory)

          0  sorts (disk)

      71806  rows processed

session2:

刚连会话之时

SQL> select event,t.TOTAL_WAITS from v$session_event t where sid=18;

EVENT                                                            TOTAL_WAITS

---------------------------------------------------------------- -----------

Disk file operations I/O                                                   1

db file sequential read                                                   11

SQL*Net message to client                                                 14

SQL*Net message from client                                               13

events in waitclass Other                                                  1

清除缓存之前

SQL> select event,t.TOTAL_WAITS from v$session_event t where sid=18;

EVENT                                                            TOTAL_WAITS

---------------------------------------------------------------- -----------

Disk file operations I/O                                                   3

log file sync                                                              3

db file sequential read                                                  573

db file scattered read                                                    51

direct path write                                                          4

SQL*Net message to client                                                 24

SQL*Net message from client                                               23

events in waitclass Other                                                  8

执行查询之后

SQL> select event,t.TOTAL_WAITS from v$session_event t where sid=18;

EVENT                                                            TOTAL_WAITS

---------------------------------------------------------------- -----------

Disk file operations I/O                                                   4

log file sync                                                              3

db file sequential read                                                  579

db file scattered read                                                   116

direct path write                                                          4

SQL*Net message to client                                               4819

SQL*Net message from client                                             4818

events in waitclass Other                                                  9

session3:

select /*+index(t ind_t_id)*/ object_id

from

 test t where object_id > 1

call     count       cpu    elapsed       disk      query    current        rows

------- ------  -------- ---------- ---------- ---------- ----------  ----------

Parse        1      0.00       0.01          2          2          0           0

Execute      1      0.00       0.00          0          0          0           0

Fetch     4789      0.14       0.23        160       4936          0       71806

------- ------  -------- ---------- ---------- ---------- ----------  ----------

total     4791      0.14       0.24        162       4938          0       71806

Misses in library cache during parse: 1

Optimizer mode: ALL_ROWS

Parsing user id: 84 

Rows     Row Source Operation

-------  ---------------------------------------------------

  71806  INDEX RANGE SCAN IND_T_ID (cr=4936 pr=160 pw=0 time=37954 us cost=173 size=819741 card=63057)(object id 73406)

Elapsed times include waiting on following events:

  Event waited on                             Times   Max. Wait  Total Waited

  ----------------------------------------   Waited  ----------  ------------
  db file sequential read                         2        0.00          0.01

  SQL*Net message to client                    4789        0.00          0.00

  SQL*Net message from client                  4789        0.00          0.18

  db file scattered read                         20        0.02          0.04

********************************************************************************