This parameter is useful when you wanted get the approximate size of the dump file. Suppose you have only limited space available in the disk and if you wanted to check whether your dump will be fit into the available space then you can use this parameter to check or estimate the size of the dump with doing the actual export. Some facts are here about estimate_only parameter.
· You don’t need specify any of other parameters like directory, dumpfile etc.
· Compression parameter will not consider for the estimate. Which means the estimate size would be same for both with and without compression parameter.
See one example
$ expdp schemas=scott estimate_only=y
Export: Release 126.96.36.199.0 - Production on Thu Feb 20 12:52:41 2014
Copyright (c) 1982, 2011, Oracle and/or its affiliates. All rights reserved.
Username: / as sysdba
Connected to: Oracle Database 11g Release 188.8.131.52.0 - 64bit Production
Starting "SYS"."SYS_EXPORT_SCHEMA_01": /******** AS SYSDBA schemas=scott estimate_only=y
Estimate in progress using BLOCKS method...
Processing object type SCHEMA_EXPORT/TABLE/TABLE_DATA
. estimated "SCOTT"."HINT_TEST" 62 MB
. estimated "SCOTT"."OBJ_LIST" 2 MB
. estimated "SCOTT"."DEPT" 64 KB
. estimated "SCOTT"."MV_DEPT" 64 KB
. estimated "SCOTT"."dept" 64 KB
Total estimation using BLOCKS method: 64.18 MB
Job "SYS"."SYS_EXPORT_SCHEMA_01" successfully completed at 12:53:09