Why fragmentation in T1 is huge and in T2 is zero

Hi
I have a busy DB 8107 , with compatible 8.0.5.0.0 on Windows 2000 SP4
(i am going to 11g very soon)
i have 2 tables , t1 in tablespace1 and t2 in tablespace2
both T1 and T2 are getting lots of inserts and deletes ,there are huge fragmentation in T1 but not T2
tablespace1 and tablspace2 are identical in definition
EX:
CREATE TABLESPACE tablespace1 DATAFILE
LOGGING
DEFAULT STORAGE (
INITIAL 104K
NEXT 104K
MINEXTENTS 1
MAXEXTENTS UNLIMITED
PCTINCREASE 0
ONLINE
PERMANENT
EXTENT MANAGEMENT DICTIONARY;
Giving no maintenance has been done to T1 or T2 or their 2 tablespaces in the last 24 months
My question is why there are huge fragmentation in t1 but not t2?
Thanks
Edited by: user8803475 on Aug 21, 2012 11:14 PM

Think about how the rows are being inserted and deleted.
In general, there are two extremes in the way that data is inserted and removed from a table.
The first way is usually chronological and sees the data getting inserted into the table, sitting there for a while then getting deleted.
In this scenario, rows have a consistent lifespan and the datablocks will in general, get filled then get emptied.
Think of a log table that is retained for 1 year. The rows get inserted and deleted using the same time based criteria.
These data blocks will get filled, get emptied and get reused pretty regularly.
Fragmentation will be near zero.
The second way is more random. The rows get inserted and deleted on entirely different criteria.
Think of a table in which rows get inserted based on time, but get deleted based on some other value in the data.
These data blocks will be filled with inserts, but partially emptied with the deletes.
The database will reuse the space when it falls below a threshold but will otherwise be "fragmented"
This explains the "why did it get this way"
Now, the part about should you "fix" it gets more to the point.
Is the "fragmentation" a problem? probably not.
Disk space is pretty cheap and your time is not.
You say that this is a pretty busy database. Unless your fragmented table has been configured with a bad PCTFREE and PCTUSED setting, the database will reuse the space.
Is it worth your time to tweak the use/reuse of small amounts of disk space?
An Oracle database does not require special maintenance procedures to reuse disk space.
The management of this space is explained very well in the database concepts guide:
http://docs.oracle.com/cd/E11882_01/server.112/e25789/logical.htm#i13690

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