使用expdp导出命令时，通常需要评估导出文件的大小来作为空间是否足够的依据，
特别是在做全库导出的场景中。
设置参数estimate_only=y|n 来指定是否仅做评估而不导数据
设置参数estimate=statistic 根据最近统计值估算对象占用空间
测试过程如下：
[oracle@hisdb1:/home/oracle]$ expdp \'/ as sysdba\' full=y estimate_only=y estimate=statistics

Export: Release 11.2.0.4.0 - Production on Fri Feb 18 16:20:27 2022

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,
Data Mining and Real Application Testing options
Starting "SYS"."SYS_EXPORT_FULL_01":  "/******** AS SYSDBA" full=y estimate_only=y estimate=statistics
Estimate in progress using STATISTICS method...

.  estimated "SYSTEM"."REPCAT$_RESOLUTION_STATISTICS"        0 KB
.  estimated "SYSTEM"."REPCAT$_RESOL_STATS_CONTROL"          0 KB
.  estimated "SYSTEM"."REPCAT$_RUNTIME_PARMS"                0 KB
.  estimated "SYSTEM"."REPCAT$_SITES_NEW"                    0 KB
.  estimated "SYSTEM"."REPCAT$_SITE_OBJECTS"                 0 KB
.  estimated "SYSTEM"."REPCAT$_SNAPGROUP"                    0 KB
.  estimated "SYSTEM"."REPCAT$_TEMPLATE_OBJECTS"             0 KB
.  estimated "SYSTEM"."REPCAT$_TEMPLATE_PARMS"               0 KB
.  estimated "SYSTEM"."REPCAT$_TEMPLATE_REFGROUPS"           0 KB
.  estimated "SYSTEM"."REPCAT$_TEMPLATE_SITES"               0 KB
.  estimated "SYSTEM"."REPCAT$_TEMPLATE_TARGETS"             0 KB
.  estimated "SYSTEM"."REPCAT$_USER_AUTHORIZATIONS"          0 KB
.  estimated "SYSTEM"."REPCAT$_USER_PARM_VALUES"             0 KB
.  estimated "SYSTEM"."SQLPLUS_PRODUCT_PROFILE"              0 KB
Total estimation using STATISTICS method: 29.25 MB
Job "SYS"."SYS_EXPORT_FULL_01" successfully completed at Fri Feb 18 16:20:42 2022 elapsed 0 00:00:14