怎么查询oracle表空间大小

SELECT tablespace_name,
ROUND(SUM(bytes) / 1024 / 1024, 2) AS "Size(MB)",
ROUND(SUM(bytes - freespace) / 1024 / 1024, 2) AS "Used(MB)",
ROUND(SUM(freespace) / 1024 / 1024, 2) AS "Free(MB)"
FROM (SELECT df.tablespace_name,
df.bytes,
SUM(fs.bytes) AS "freespace"
FROM (SELECT tablespace_name,
ROUND(SUM(bytes) / 1048576) AS bytes
FROM dba_data_files
GROUP BY tablespace_name) df,
(SELECT tablespace_name,
ROUND(SUM(bytes) / 1048576) AS bytes
FROM dba_free_space
GROUP BY tablespace_name) fs
WHERE df.tablespace_name = fs.tablespace_name(+)
GROUP BY df.tablespace_name, df.bytes)
GROUP BY tablespace_name;