Difference between LOB segement size and DBMS_LOB.GETLENGTH

Question

SQL> select bytes/1024/1024 from dba_segments where segment_name='SYS_LOB0000130003C00019$$';
BYTES/1024/1024
14772
SQL> select TABLE_NAME,COLUMN_NAME from dba_lobs where SEGMENT_NAME='SYS_LOB0000130003C00019$$';
TABLE_NAME
COLUMN_NAME
TBL
C1
SQL> select sum(DBMS_LOB.GETLENGTH(C1))/1024/1024 from TBL;
SUM(DBMS_LOB.GETLENGTH(C1)
30.0376911
why is there a discrepancy between the two sizes (14GB and 30MB).
Here are the storage characteristics from the TBL ddl for the C1 LOB column:
TABLESPACE TBLSPC ENABLE STORAGE IN ROW CHUNK 8192 PCTVERSION 10
NOCACHE LOGGING
STORAGE(INITIAL 1048576 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT))

Answer 1

user10508599 wrote:
why is there a discrepancy between the two sizes (14GB and 30MB).
Here are the storage characteristics from the TBL ddl for the C1 LOB column:
TABLESPACE TBLSPC ENABLE STORAGE IN ROW CHUNK 8192 PCTVERSION 10
NOCACHE LOGGING
STORAGE(INITIAL 1048576 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645
PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT))
According to your storage parameters it only requires 14772 rows that have lob values that are stored out of row, i.e. larger than approx. 4k. For each lob segment 1M will be allocated at least, so that might a reasonable explanation.
Not sure where I had my mind when writing this, but this is certainly wrong (and no one complaining...).
You could have a lot of deleted rows in the table, or LOBs that were larger but shrinked now, or you could hit a storage allocation bug.
What is the block size of the tablespace for the LOBs?
Regards,
Randolf
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Edited by: Randolf Geist on Jun 16, 2009 7:20 AM
Corrected plain wrong statement