When you export to dump file with compress=y, EXPORT utility modifies the INITIAL value of the table to be the sum of the current extents. So that means if the table has 50 * 10K extents filled, then the INITIAL extent when you IMPORT the data will be 500K.
thanks , i very well know that this is what happens, but what i was wondering is that how do one come to know that such a option is used,, any query , DD views to tell us that compress =y has taken effect
if you understand well delete, truncate, segments and extents you will see why
if I have 100 extents with 1MB each but there are only 1MB real data what happens if I export with compress=y? I will get one 100MB extent 99% empty, if I use compress=n what happens? I get 1extent of 1MB, I saved 99MB