The value for parameter ESTIMATE  is either BLOCKS (default) or STATISTICS.

BLOCKS: The estimated space used is calculated by multiplying the number of database blocks used by the target objects with the appropriate block sizes. It is  the least accurate because of:

  • The table was created with a much bigger initial extent size than was needed for the actual table data
  • Many rows have been deleted from the table, or a very small percentage of each block is used.

STATISTICS: The estimated space used is calculated by using statistics for each table. If the table has been recently analyzed, the “estimate=statistics” would be the most accurate.

If a table has LOBs, ESTIMATE dump file size does NOT take LOB size into consideration.

Note that the outcome specified by ESTIMATE=BLOCKS is far away from the size of the actual dumpfile. In fact, ESTIMATE=BLOCKS method generates more inaccurate result from dump file size when,

  • The table was created with a much bigger initial extent size than was needed for the actual table data.
  • Many rows have been deleted from the table, or a very small percentage of each block is used.
    The outcome generated by ESTIMATE=STATISTICS is most accurate to dump file size if recently table is analyzed.

Below is an example shown both in case of ESTIMATE=STATISTICS and ESTIMATE=BLOCKS. In both cases data pump export dump file is generated after estimation of dump file.

Now we will export the schemas by using ESTIMATE=STATISTICS

Using ESTIMATE=BLOCKS, before data pump export size is shown as 448 KB and using ESTIMATE=STATISTICS, before data pump export size is shown as 55.95 KB and my actual dump file size was 408KB which is away from estimation using ESTIMATE=BLOCKS as difference is 448-408=40 KB. In later case difference is 448-55.95=392.05KB.

Thank you for giving your valuable time to read the above information.

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