以下方式进行预估
SQL> select sum(bytes)/1024/1024 from v$datafile; SUM(BYTES)/1024/1024 -------------------- 990 SQL> select sum(bytes)/1024/1024 from dba_free_space; SUM(BYTES)/1024/1024 -------------------- 344.25
预估大小为645GB 990-344.25=645.75 Mb
说明：如果是压缩备份，压缩比一般为2-4倍，备份过程也应考虑归档备份空间