LOB segment size is 2 times bigger than the real data

Question

here's an interesting test:
1. I created a tablespace called "smallblock" with 2K blocksize
2. I created a table with a CLOB type field and specified the smallblock tablespace as a storage for the LOB segment:
SCOTT@andrkydb> create table t1 (i int, b clob) lob (b) store as
t1_lob (chunk 2K disable storage in row tablespace smallblock);
3. I insert data into the table, using a bit less than 2K of data for the clob type column:
SCOTT@andrkydb> begin
2 for i in 1..1000 loop
3 insert into t1 values (mod(i,5), rpad('*',2000,'*'));
4 end loop;
5 end;
6 /
4. Now I can see that I have an average of 2000 bytes for each lob item:
SCOTT@andrkydb> select avg(dbms_lob.getlength(b)) from t1;
AVG(DBMS_LOB.GETLENGTH(B))
2000
and that all together they take up:
SCOTT@andrkydb> select sum(dbms_lob.getlength(b)) from t1;
SUM(DBMS_LOB.GETLENGTH(B))
2000000
But when I take a look at how much is the LOB segment actually taking, I get a result, which is being a total mystery to me:
SCOTT@andrkydb> select bytes from dba_segments where segment_name = 'T1_LOB';
BYTES
5242880
What am I missing? Why is LOB segment is being ~2 times bigger than it is required by the data?
I am on 10.2.0.3 EE, Solaris 5.10 sparc 64bit.
Message was edited by:
Andrei Kübar

Answer 1

thanks for the link, it is good to know such thing is possible. Although I don't really see how can it help me..
But you know, you were right regarding the smaller data amounts. I have tested with 1800 bytes of data and in this case it does fit just right.
But this means that there is 248 bytes wasted (from my, as developer, point of view) per block! But if there is such an overhead, then I must be able to estimate it when designing the data structures. And I don't see anywhere in the docs a single word about such thing.
Moreover, if you use NCLOB type, then only 990 bytes fits into a single 2K chunk. So the overhead might become really huge when you go over to gigabyte amounts...
I have a LOB segment for a nclob type field in a production database, which is 5GB large and it contains only 2,2GB of real data. There is no "deleted" rows in it, I know because I have rebuilt it. So this looks like a total waste of disk space... I must say, I'm quite disappointed with this.
- Andrei