导读：
今天在做实验的时候发现：truncate table后x$bh不会再记录对此table块的访问，具体实验如下：
SQL>create table t3 ( a number);
Table created.
SQL>select object_id from dba_objects where object_name='T3';
OBJECT_ID
----------
51560
SQL>select obj,tch,dbarfil,dbablk from x$bh where obj=51560;
OBJ TCH DBARFIL DBABLK
---------- ---------- ---------- ----------
51560 2 1 60465
SQL>insert into t3 select level from dual connect by level<=1000;
1000 rows created.
SQL>select obj,tch,dbarfil,dbablk from x$bh where obj=51560;
OBJ TCH DBARFIL DBABLK
---------- ---------- ---------- ----------
51560 1 1 60467
51560 1 1 60466
51560 3 1 60465
SQL>truncate table t3;
Table truncated.
SQL>select obj,tch,dbarfil,dbablk from x$bh where obj=51560;
no rows selected
SQL>insert into t3 select level from dual connect by level<=1000;
1000 rows created.
SQL>commit;
Commit complete.
SQL>select obj,tch,dbarfil,dbablk from x$bh where obj=51560;
no rows selected
但是如果table中没有任何数据（只有一个块记录段头）时，x$bh就不会受truncate影响
SQL>create table t4 (a number);
Table created.
SQL>select object_id from dba_objects where object_name='T4';
OBJECT_ID
----------
51566
SQL>select obj,tch,dbarfil,dbablk from x$bh where obj=51566;
OBJ TCH DBARFIL DBABLK
---------- ---------- ---------- ----------
51566 1 1 60473
SQL>truncate table t4;
Table truncated.
SQL>select obj,tch,dbarfil,dbablk from x$bh where obj=51566;
OBJ TCH DBARFIL DBABLK
---------- ---------- ---------- ----------
51566 2 1 60473
本文转自
http://woodnan.itpub.net/post/37055/457768
转载于:https://www.cnblogs.com/myitworld/archive/2008/03/24/2214901.html
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