V$FILESPACE_USAGE 查询结果无数据

V$FILESPACE_USAGE summarizes space allocation information of each datafile and tempfile.

视图查无数据原因：数据库达到db_files参数限制

show parameter db_files
NAME     TYPE    VALUE
-------- ------- ------
db_files integer 9

select max(file#) from v$datafile;
MAX(FILE#)
----------
         8

SYS@MYDB(mydb): 1> SELECT * from DBA_TABLESPACE_USAGE_METRICS;
no rows selected << No Rows returned

SYS@MYDB(mydb): 1> SELECT * FROM V$FILESPACE_USAGE;
no rows selected << No Rows returned 


CAUSE
Issue is due to Bug 18921384 - DBA_TABLESPACE_USAGE_METRICS VIEW DOES NOT RETURN ROWS WHEN DB_FILES LIMIT IS REACHED
Which is closed as a duplicate of Bug 16317020 - DBA_TABLESPACE_USAGE_METRICS HAS NO RECORD AFTER CONTROFILE RECREATED 

SOLUTION
Download and apply Patch 16317020 if available for your platform. The Bugfix is included in 12.1.0.1 onwards. 

调整参数，重启数据库，再查询时已经有数据了

SYS@MYDB(mydb): 1> alter system set db_files=20 scope=spfile;
SYS@MYDB(mydb): 1> shutdown immediate;
SYS@MYDB(mydb): 1> startup
Database opened.
SYS@MYDB(mydb): 1> SELECT * FROM V$FILESPACE_USAGE;

TABLESPACE_ID       RFNO ALLOCATED_SPACE  FILE_SIZE FILE_MAXSIZE CHANGESCN_BASE CHANGESCN_WRAP       FLAG
------------- ---------- --------------- ---------- ------------ -------------- -------------- ----------
            0          1           95776      98560      4194302        3379215              0          2
            1          2           91264      96000      4194302        3379215              0          2
            2          1             160      12800       447830        3379215              0          6
            3          1             128       3712       438742        3379215              0          6
            4          4             768       9120      4194302        3379215              0          2
            6          5            2600      25600        25600        3379215              0          2
            7          6      4294966704       1280         1280        3379215              0          2
            8          7             128      12800        12800        3379215              0          2
           10          8             208       1280         1280        3379215              0          2

参考：DBA_TABLESPACE_USAGE_METRICS View Returns No Rows When db_files Limit Is Reached (Doc ID 1903251.1)