【聚杰网备份与恢复】library cache pin与PROCEDURE的重建

    我们看一下以下测试，首先在第一个session执行操作：

SQL> create or replace PROCEDURE pining
2 IS
3 BEGIN
4 NULL;
5 END;
6 /

Procedure created.

SQL>
SQL> alter session set nls_date_format='yyyy-mm-dd hh24:mi:ss';

Session altered.

SQL> create or replace procedure calling
2 is
3 begin
4 pining;
5 dbms_lock.sleep(60);
6 end;
7 /

Procedure created.

SQL>
SQL> col object_name for a30
SQL> select object_name,last_ddl_time from dba_objects where object_name in ('PINING','CALLING');

OBJECT_NAME LAST_DDL_TIME
------------------------------ -------------------
CALLING 2007-04-02 09:12:57
PINING 2007-04-02 09:12:57

SQL>
SQL> exec calling;

    此时Calling对于Pining的引用将会在Pining的Body上获得共享Pin，此时在另外一个Session执行重建Procedure的操作：

SQL> create or replace PROCEDURE pining
2 IS
3 BEGIN
4 NULL;
5 END;
6 /

    这个操作将一直挂起，直到第一个session的操作完成，此时在第三个session可以观察到Library Cache Pin的竞争：

SQL> select sid,event from v$session where username='EYGLE'
2 /

SID EVENT
---------- ----------------------------------------------------------------
137 library cache pin
139 PL/SQL lock timer
157 SQL*Net message to client

     当第一个session执行完成之后，第二个session的操作随之完成，我们可以看到LAST_DDL_TIME并未改变：

SQL> exec calling;

PL/SQL procedure successfully completed.

SQL>
SQL> select object_name,last_ddl_time from dba_objects where object_name in ('PINING','CALLING');

OBJECT_NAME LAST_DDL_TIME
------------------------------ -------------------
CALLING 2007-04-02 09:12:57
PINING 2007-04-02 09:12:57