ORACLE从共享池删除指定SQL的执行计划

2017-01-04 10:32:11来源:作者:潇湘隐者人点击

Oracle 11g在DBMS_SHARED_POOL包中引入了一个名为PURGE的新存储过程,用于从对象库缓存中刷新特定对象,例如游标,包,序列,触发器等。也就是说可以删除、清理特定SQL的执行计划,这样在特殊情况下,就避免你要将整个SHARED POOL清空的危险情况。例如某个SQL语句由于优化器产生了错误的执行计划,我们希望优化器重新解析,生成新的执行计划,必须先将SQL的执行计划从共享池中刷出或将其置为无效,那么优化器才能将后续SQL进行硬解析、生成新的执行计划。这在以前只能使用清空共享池的方法。现在就可以指定刷新特定SQL的执行计划。当然在10.2.0.4 和10.2.0.5的补丁集中该包也被包含进来,该包的存储过程有三个参数,如下所示:

DBMS_SHARED_POOL.PURGE ( name VARCHAR2, flag CHAR DEFAULT 'P', heaps NUMBER DEFAULT 1); Argument Name Type In/Out Default? ------------------------------ ----------------------- ------ -------- NAME VARCHAR2 IN FLAG CHAR IN DEFAULT HEAPS NUMBER IN DEFAULT

第一个参数: 为逗号分隔的ADDRESS列和HASH_VALUE列的值。

第二个参数: 可以有多个选项,例如C、P、T、R、Q等。具体意义如下所示

C表示PURGE的对象是CURSOR

Set to 'P' or 'p' to fully specify that the input is the name of a package/procedure/function.

Set to 'T' or 't' to specify that the input is the name of a type.

Set to 'R' or 'r' to specify that the input is the name of a trigger.

Set to 'Q' or 'q' to specify that the input is the name of a sequence.

...................................

第三个参数: heaps,一般使用默认值1

Heaps to be purged. For example, if heap 0 and heap 6 are to be purged:

1<<0 | 1<<6 => hex 0x41 => decimal 65, so specify heaps =>65.Default is 1, that is, heap 0 which means the whole object would be purged

在ORACLE 11g当中,你可以在$ORACLE_HOME/rdbms/admin/dbmspool.sql中查看该包的具体定义. 但是这个DBMS_SHARED_POOL.PURGE在10.2.0.4.0(实际测试发现10.2.0.5.0也存在同样问题)都有一些问题,它可能无法生效,当然在Oracle 11g中没有这个问题,具体演示如下所示:

SQL> select * from v$version;BANNER----------------------------------------------------------------Oracle Database 10g Release 10.2.0.5.0 - 64bit ProductionPL/SQL Release 10.2.0.5.0 - ProductionCORE 10.2.0.5.0 ProductionTNS for Linux: Version 10.2.0.5.0 - ProductionNLSRTL Version 10.2.0.5.0 - ProductionSQL> alter system flush shared_pool;System altered.SQL> set linesize 1200;SQL> select * from scott.dept where deptno=40; DEPTNO DNAME LOC---------- -------------- ------------- 40 OPERATIONS BOSTONSQL> select sql_id, first_load_time 2 from v$sql 3 where sql_text like'select * from scott.dept%';SQL_ID FIRST_LOAD_TIME------------- ---------------------------------------------------------3nvuzqdn6ry6x 2016-12-29/08:51:21SQL> col sql_text for a64;SQL> select address, hash_value, sql_text 2 from v$sqlarea 3 where sql_id='3nvuzqdn6ry6x';ADDRESS HASH_VALUE SQL_TEXT---------------- ---------- ----------------------------------------------------------------00000000968ED510 1751906525 select * from scott.dept where deptno=40SQL> exec dbms_shared_pool.purge('00000000968ED510,1751906525','C');PL/SQLprocedure successfully completed.SQL> select address, hash_value, sql_text 2 from v$sqlarea 3 where sql_id='3nvuzqdn6ry6x';ADDRESS HASH_VALUE SQL_TEXT---------------- ---------- ------------------------------------------00000000968ED510 1751906525 select * from scott.dept where deptno=40SQL>

如上截图所示,DBMS_SHARED_POOL.PURGE并没有清除这个特定的SQL的执行计划,其实这个是因为在10.2.0.4.0 要生效就必须开启5614566 EVNET,否则不会生效。 具体可以参考官方文档:

DBMS_SHARED_POOL.PURGE Is Not Working On 10.2.0.4 (文档 ID 751876.1)

Bug 7538951 : DBMS_SHARED_POOL IS NOT WORKING AS EXPECTED

Bug 5614566 : WE NEED A FLUSH CURSOR INTERFACE

DBMS_SHARED_POOL.PURGE is available from 11.1. In 10.2.0.4, it is available

through the fix for Bug 5614566 . However, the fix is event protected. You need to set the event 5614566 to make use of purge. Unless the event is set, dbms_shared_pool.purge will have no effect.

Set the event 5614566 in the init.ora to turn purge on.

event="5614566 trace name context forever"

如下所示,设置5614566 event后,必须重启数据库才能生效,这个也是一个比较麻烦的事情。当然这也是一个没有办法的事情.

alter system set event = '5614566 trace name context forever' scope = spfile;

最新文章

123

最新摄影

微信扫一扫

第七城市微信公众平台