My database size was 1TB and its full export size is 300GB.

After imported db to new environment, db size reduced to 700GB. What could be reason for this ?

  • This happens as a matter of creating nicely contiguous extends within an Oracle database. After a while, a database gets fragmented because of updates, deletes, etc. Exporting and then importing allows data extents from data and undo to be more contiguous, show less used space.
    – Data Flux
    Apr 30, 2018 at 13:01

3 Answers 3


There could be many reasons. But generally, it may be due to:

  • Free space in data files or temporary files
  • Undo data
  • Row chaining and migrating
  • Data blocks fragmentation

Check the size of your temp tablespace and the redo tablespace. The reduction may come from there.


This is exactly what I would expect of any good RDBMS.

A database that has been used for a long amount of time will fragment. Your efficiency of the fragmented database is 70%, which I would say is pretty good.

When importing the data into a fresh database, there will be no fragmentation, but after you have deleted and inserted lots of rows, you will see fragmentation will hit you again.

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