10200(consistent read buffer status)内部诊断事件可以用于探测一致性读CR(consistent read)块的访问情况，虽然cr读的统计信息可以从v$sysstat或AWR/statspack中获取，但是10200 event还是我们研究Consistent Read一致性读的有力工具。该事件可以通过在会话session级别设置ALTER SESSION SET EVENTS 或 DBMS_SYSTEM.SET_EV. Set 来开启，一般调用级别为Level 10。
该事件返回的trace跟踪文件，记录了为了实现一致性读的目的，哪些数据块以及这些块的各历史版本在执行过程中被创建(CR block creation)并检验(CR block inspection)，以找出Best CR block满足Consistent一致性。
注意10200 Internal Event主要是被ktrgtc和ktrget(call ktrget to get one block ->calling KTR layer to apply RBS to have consistent read Block;)这2个Oracle内核功能函数触发，这2个内部函数Internal Function的主要作用：
ktrget:
- Initializes a buffer cache CR scan request
- Calls kcbgtcr for the best resident buffer to start from to build the CR buffer
- Calls ktrgcm to build the CR buffer by applying undo
- Returns CR buffer to the requestor
- If successful, returns the “best” candidate (performed by ktrexf or examination function)
- Scans the hash bucket for the DBA for buffers that may be used to build a CR buffer
- If not successful, calls kcbget
[oracle@rh2 ~]$ sqlplus / as sysdba
SQL*Plus: Release 10.2.0.4.0 - Production on Fri Sep 30 21:23:47 2011
Copyright (c) 1982, 2007, Oracle. All Rights Reserved.
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing option
SQL> select * from global_name;
GLOBAL_NAME
--------------------------------------------------------------------------------
www.oracledatabase12g.com & www.askmaclean.com
SQL> create table tv(t1 int);
Table created.
SQL> alter session set events '10200 trace name context forever,level 10';
Session altered.
SQL> select * from tv;
T1
----------
1
10200 trace
Consistent read started for block 0 : 0040081a
