求sql优化

SELECT UPPER(F.TABLESPACE_NAME) tablespacename,--"表空间名",
D.TOT_GROOTTE_MB tablespacesize,--"表空间大小(M)",
D.TOT_GROOTTE_MB - F.TOTAL_BYTES used,--"已使用空间(M)",
TO_CHAR(ROUND((D.TOT_GROOTTE_MB - F.TOTAL_BYTES) / D.TOT_GROOTTE_MB * 100,2),'990.99') || '%' usages, --"使用比",
F.TOTAL_BYTES --"空闲空间(M)",
FROM (
SELECT TABLESPACE_NAME,
ROUND(SUM(BYTES) / (1024 * 1024), 2) TOTAL_BYTES
FROM SYS.DBA_FREE_SPACE
GROUP BY TABLESPACE_NAME
) F,
(
SELECT DD.TABLESPACE_NAME,
ROUND(SUM(DD.BYTES) / (1024 * 1024), 2) TOT_GROOTTE_MB
FROM SYS.DBA_DATA_FILES DD
GROUP BY DD.TABLESPACE_NAME
) D

WHERE D.TABLESPACE_NAME = F.TABLESPACE_NAME
ORDER BY 1