How does Oracle determine Segment allocation in LMT with autoallocate?

  I’ve noticed in recent months that a certain table, when it grows, is growing by a very large segment size.  This table is located in an LMT tablespace using autoallocate. By virtue of this there is no next_extent or pctincrease value (both values are null at the table and tablespace level).

 

Last week Friday between Midnight and 2 am the table grew by 800 Mb from 6.9 Gb (292 extents) to 7.7 GB (304 extents).  Using dba_hist_snapshot and dba_hist_seg_stat I retrieved the following data:

 

Date HH24     space_used_total     space_allocated_total

06/06/09 00            1096035799         1006632960

06/06/09 02            1110456421         1811939328

 

While these numbers do not reflect the actual table size (7.7 GB), it does illustrate the large jump in size on the allocation versus the disproportionate growth on the used size.