Why is my index bigger than my table ?

Question

Hi
I am using locally managed tablespaces, with system allocation on 9.2.0.8
I have a table with some 32 columns and a 3 column index (unique key).
The 3 columns indexed are not null and all number(12) datatype.
The index is bigger than the table by a factor of almost 10.
I am not using compression of any kind.
Any ideas ?
Cheers

Answer 1

If the table has insert or delete activity on it then the index is being maintained and it is this maintenance that can lead to an index growing larger than the base table when the length of the indexed columns are small compared to the row length.
If one of the 3 key columns is a sequence number or generally follows an ever increasing value pattern and the delete process does not delete all values within the leaf blocks then the index will have a tendency to grow large realtive to the table.
Worse is if any of the three key values are allowed to be updated after insertion. That then becomes a delete and insert into the index.
If the tablespace was originally a dictionary managed tablespace and was converted to locally managed auto-allocate via sys.dbms_space_admin.tablespace_migrate_to_local then the storage behavior is different from if the tablespace was created auto-allocate from day one.
Calculate the expected size of the index. Rebuild or re-create it as free space ditakes and then compare the resulting size to the expected size.
HTH -- Mark D Powell --