Update 2016-04-26: this is new bug 25954054 – WRH$_SGASTAT_U BECOMING UNUSABLE STATE IN UPGRADED DB.

After upgrading our OMR and AWRW instances from 12.1 to 12.2, our monitoring revealed that index SYS.WRH$_SGASTAT_U was becoming unusable at regular intervals.
The first couple of times, we just rebuilt it in the hope that the problem would just go away(!); of course it didn’t, so I had to do a little bit of investigation.

Reviewing the alert log identified the exact time that the problem had occurred:

2017-04-12T08:11:43.674485+08:00
Some indexes or index [sub]partitions of table SYS.WRH$_SGASTAT have been marked unusable

Grepping for “SGASTAT” in the trace directory, also revealed that the index being unusable was causing inserts into AWR had been failing:

*** 2017-04-12T12:00:18.338061+08:00
*** SESSION ID:(289.8864) 2017-04-12T12:00:18.338092+08:00
*** CLIENT ID:() 2017-04-12T12:00:18.338103+08:00
*** SERVICE NAME:(SYS$BACKGROUND) 2017-04-12T12:00:18.338108+08:00
*** MODULE NAME:(MMON_SLAVE) 2017-04-12T12:00:18.338119+08:00
*** ACTION NAME:(Auto-Flush Slave Action) 2017-04-12T12:00:18.338125+08:00
*** CLIENT DRIVER:() 2017-04-12T12:00:18.338132+08:00

*** KEUUXS - encountered error: (ORA-01502: index 'SYS.WRH$_SGASTAT_U' or partition of such index is in unusable state
)
  *** SQLSTR: total-len=464, dump-len=464,
      STR={insert into  wrh$_sgastat  (dbid, per_pdb, con_dbid, snap_id, instance_number, pool, name, bytes)  select distinct  :dbid, 0 per_pdb,  :srcdbid  con_dbid,  :snap_id, :instance_number, pool, name, bytes FROM     (select con_id, pool, name, bytes,             100*(bytes) / (sum(bytes) over (partition by pool)) part_pct        from  v$sgastat      ) sgastat  WHERE (1 = 1)   and part_pct >= 1      or pool is null      or name = 'free memory' or name = 'row cache' }
*** KEWRAFM1: Error=13509 encountered by kewrfteh

As we still had the redo logs for the period of time identified by the alert log, I fired up logminer, and right after a bunch of inserts into WRH$_SGASTAT, I found the following statement:

alter table WRH$_SGASTAT drop partition WRH$_SGASTA_1521710113_17057;

I am guessing this is part of the standard mechanism for purging AWR data based on configured retention settings.
A quick check reveals that of the partitioned AWR base tables, 3 have indexes that are not partitioned:

SQL> SELECT                                                                                                                                                                                                          
  2     partitioned,                                                                                                                                                                                                 
  3     COUNT(*)                                                                                                                                                                                                     
  4  FROM                                                                                                                                                                                                            
  5     dba_indexes                                                                                                                                                                                                  
  6  WHERE                                                                                                                                                                                                           
  7     index_type = 'NORMAL'                                                                                                                                                                                        
  8  AND                                                                                                                                                                                                             
  9     table_name IN (                                                                                                                                                                                              
 10        SELECT                                                                                                                                                                                                    
 11           table_name                                                                                                                                                                                             
 12        FROM                                                                                                                                                                                                      
 13           dba_tables                                                                                                                                                                                             
 14        WHERE                                                                                                                                                                                                     
 15           table_name LIKE 'WRH$\_%' ESCAPE '\'                                                                                                                                                                   
 16        AND                                                                                                                                                                                                       
 17           partitioned = 'YES'                                                                                                                                                                                    
 18     )                                                                                                                                                                                                            
 19  GROUP BY                                                                                                                                                                                                        
 20     partitioned;                                                                                                                                                                                                 
                                                                                                                                                                                                                     
PAR   COUNT(*)                                                                                                                                                                                                       
--- ----------                                                                                                                                                                                                       
NO           3                                                                                                                                                                                                       
YES         35                                                                                                                                                                                                       

SQL> SELECT                                                                                                                                                                                                          
  2     table_owner,                                                                                                                                                                                                 
  3     table_name,                                                                                                                                                                                                  
  4     index_name                                                                                                                                                                                                   
  5  FROM                                                                                                                                                                                                            
  6     dba_indexes                                                                                                                                                                                                  
  7  WHERE                                                                                                                                                                                                           
  8     index_type = 'NORMAL'                                                                                                                                                                                        
  9  AND                                                                                                                                                                                                             
 10     table_name IN (                                                                                                                                                                                              
 11        SELECT                                                                                                                                                                                                    
 12           table_name                                                                                                                                                                                             
 13        FROM                                                                                                                                                                                                      
 14           dba_tables                                                                                                                                                                                             
 15        WHERE                                                                                                                                                                                                     
 16           table_name LIKE 'WRH$\_%' ESCAPE '\'                                                                                                                                                                   
 17        AND                                                                                                                                                                                                       
 18           partitioned = 'YES'                                                                                                                                                                                    
 19     )                                                                                                                                                                                                            
 20  AND partitioned = 'NO';                                                                                                                                                                                         
                                                                                                                                                                                                                     
TABLE_OWNER                    TABLE_NAME                     INDEX_NAME                                                                                                                                             
------------------------------ ------------------------------ ------------------------------                                                                                                                         
SYS                            WRH$_SGASTAT                   WRH$_SGASTAT_U                                                                                                                                         
SYS                            WRH$_CON_SYSMETRIC_HISTORY     WRH$_CON_SYSMET_HISTORY_INDEX                                                                                                                          
SYS                            WRH$_SYSMETRIC_HISTORY         WRH$_SYSMETRIC_HISTORY_INDEX                                                                                                                           

Running the same query on both 11.2 and 12.1 instances reveals no such mis-configured indexes We are following up with Oracle Support, but I would be interested to know if anyone else can check my findings on their 12.2 instances.

Working with Oracle Support we have worked around the issue using the following script:

ALTER TABLE wrh$_sgastat DROP CONSTRAINT wrh$_sgastat_u;

ALTER TABLE wrh$_sgastat ADD CONSTRAINT wrh$_sgastat_u UNIQUE ( dbid,snap_id,instance_number,name,pool,con_dbid )
   USING INDEX
      LOCAL
   TABLESPACE sysaux;

In addition, I believe I have found the bug in file $ORACLE_HOME/rdbms/admin/c1201000.sql at line 3036:

alter table wrh$_sgastat drop constraint wrh$_sgastat_u;

alter table wrh$_sgastat add constraint wrh$_sgastat_u
        unique (dbid, snap_id, instance_number, name, pool, con_dbid, stattype);