1、案例概述
客户有一套19C的CDB环境，其中一个PDB的SYSAUX表空间的空间使用竟然高达80GB左右。在以前的认知中，SYSAUX表空间最多只有十几个GB，是什么东西占用了这么大的空间呢，想弄个明白。
2、案例分析
2.1 运行awrinfo.sql脚本，先找出是哪些对象占用空间，信息如下所示。
*****************************************************
(1a) SYSAUX usage - Schema breakdown (dba_segments)
*****************************************************
|
| Total SYSAUX size 83,142.0 MB ( 74% of 112,636.0 MB MAX with AUTOEXTEND OFF )
|
| Schema SYS occupies 83,064.0 MB ( 99.9% )
| Schema XDB occupies 53.9 MB ( 0.1% )
| Schema AUDSYS occupies 15.8 MB ( 0.0% )
| Schema WMSYS occupies 6.6 MB ( 0.0% )
| Schema GSMADMIN_INT occupies 1.4 MB ( 0.0% )
| Schema SYSTEM occupies 0.3 MB ( 0.0% )
| Schema DBSNMP occupies 0.2 MB ( 0.0% )
......
**********************************
(4) Space usage by non-AWR components (> 500K)
**********************************
COMPONENT MB SEGMENT_NAME SEGMENT_TYPE
--------- --------- --------------------------------------------------------------------- ---------------
NON_AWR 32,660.0 SYS.EXP_HEAD$ TABLE
NON_AWR 23,712.2 SYS.SYS_LOB0000011147C00004$$ LOBSEGMENT
NON_AWR 14,657.0 SYS.I_EXP_HEAD$ INDEX
NON_AWR 1,632.0 SYS.WRI$_ADV_OBJECTS TABLE
NON_AWR 1,520.0 SYS.WRI$_SQLSET_PLANS TABLE
NON_AWR 936.2 SYS.SYS_LOB0000009222C00039$$ LOBSEGMENT
NON_AWR 762.0 SYS.WRI$_ADV_OBJECTS_IDX_02 INDEX
NON_AWR 750.0 SYS.WRI$_ADV_OBJECTS_IDX_01 INDEX
NON_AWR 584.0 SYS.WRI$_OPTSTAT_HISTHEAD_HISTORY.SYS_P181 TABLE PARTITION
NON_AWR 504.0 SYS.I_WRI$_OPTSTAT_HH_OBJ_ICOL_ST INDEX
NON_AWR 426.0 SYS.WRI$_ADV_OBJECTS_PK INDEX
NON_AWR 395.0 SYS.WRI$_SQLSET_PLAN_LINES TABLE
NON_AWR 378.0 SYS.WRI$_SQLSET_STATISTICS TABLE
NON_AWR 376.0 SYS.I_WRI$_OPTSTAT_HH_ST INDEX
NON_AWR 321.0 SYS.WRI$_ADV_SQLT_PLAN_HASH TABLE
NON_AWR 248.0 SYS.WRI$_SQLSET_STATEMENTS TABLE
NON_AWR 240.0 SYS.WRI$_ADV_SQLT_PLAN_HASH_01 INDEX
NON_AWR 190.0 SYS.WRI$_SQLSET_MASK_PK INDEX
NON_AWR 176.0 SYS.WRI$_SQLTEXT_REFCOUNT_PK INDEX
NON_AWR 169.0 SYS.WRI$_ADV_SQLT_PLAN_STATS TABLE
可以看出，SYS.EXP_HEAD$这张表是元凶， 这张表是干什么用的呢？ 以前对它没什么了解。只能查查ORACLE官方文档了。
2.2 根据文章《Expression Tracking Feature Cannot Be Turned Off By OPTIMIZER_FEATURES_ENABLE(KB132474)》和《DMU reporting unconvertible data for EXP_HEAD$(KB132412)》的介绍，sys.exp_head$是12.2版本中引入的优化器新特性，用于收集SQL查询中的表达式使用统计信息。 SYS.EXP_HEAD$表中的数据如何处理？根据另外一篇internal的文章，可以关闭该特性，并且清空该对象中的数据，具体如下。
3、案例处理
Disable future monitoring:
alter system set "_column_tracking_level"=17 scope=both;
--wait 10 minutes
To purge data from tables:
exec DBMS_STATS.FLUSH_DATABASE_MONITORING_INFO() ;
truncate table sys.exp_head$ drop storage;
truncate table sys.exp_obj$ drop storage;
truncate table sys.exp_stat$ drop storage;
alter index SYS.I_EXP_HEAD$ rebuild tablespace sysaux online;
alter index SYs.I_EXP_STAT$ rebuild tablespace sysaux online;
alter index sYs.I_EXP_OBJ$ rebuild tablespace sysaux online;
To re-enable expression statistics monitoring later :
alter system set "_column_tracking_level"=21 scope=both;
Starting in 19c，we would change this to 53 since that is the default value for this version higher
alter system set "_column_tracking_level"=53 scope=both;