Index size increases than table size

Question

Hi All,
Let me know what are the possible reasons for index size greater than the table size and in some cases index size smaller than table size . ASAP
Thanks in advance
sherief

Answer 1

hi,
The size of a index depends how inserts and deletes occur.
With sequential indexes, when records are deleted randomly the space will not be reused as all inserts are in the leading leaf block.
When all the records in a leaf blocks have been deleted then leaf block is freed (put on index freelist) for reuse reducing the overall percentage of free space.
This means that if you are deleting aged sequence records at the same rate as you are inserting, then the number of leaf blocks will stay approx constant with a constant low percentage of free space. In this case it is most probably hardly ever worth rebuilding the index.
With records being deleted randomly then, the inefficiency of the index depends on how the index is used.
If numerous full index (or range) scans are being done then it should be re-built to reduce the leaf blocks read. This should be done before it significantly affects the performance of the system.
If index access’s are being done then it only needs to be rebuilt to stop the branch depth increasing or to recover the unused space
here is a exemple how index size can become larger than table size:
Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.3.0
Connected as admin
SQL> create table rich as select rownum c1,'Verde' c2 from all_objects;
Table created
SQL> create index rich_i on rich(c1);
Index created
SQL> select segment_type,bytes,blocks,extents from user_segments where segment_name like 'RICH%';
SEGMENT_TYPE BYTES BLOCKS EXTENTS
TABLE 1179648 144 9
INDEX 1179648 144 9
SQL> delete from rich where mod(c1,2)=0;
29475 rows deleted
SQL> commit;
Commit complete
SQL> select segment_type,bytes,blocks,extents from user_segments where segment_name like 'RICH%';
SEGMENT_TYPE BYTES BLOCKS EXTENTS
TABLE 1179648 144 9
INDEX 1179648 144 9
SQL> insert into rich select rownum+100000, 'qq' from all_objects;
58952 rows inserted
SQL> commit;
Commit complete
SQL> select segment_type,bytes,blocks,extents from user_segments where segment_name like 'RICH%';
SEGMENT_TYPE BYTES BLOCKS EXTENTS
TABLE 1703936 208 13
INDEX 2097152 256 16
SQL> insert into rich select rownum+200000, 'aa' from all_objects;
58952 rows inserted
SQL> select segment_type,bytes,blocks,extents from user_segments where segment_name like 'RICH%';
SEGMENT_TYPE BYTES BLOCKS EXTENTS
TABLE 2752512 336 21
INDEX 3014656 368 23
SQL> delete from rich where mod(c1,2)=0;
58952 rows deleted
SQL> commit;
Commit complete
SQL> select segment_type,bytes,blocks,extents from user_segments where segment_name like 'RICH%';
SEGMENT_TYPE BYTES BLOCKS EXTENTS
TABLE 2752512 336 21
INDEX 3014656 368 23
SQL> insert into rich select rownum+300000, 'hh' from all_objects;
58952 rows inserted
SQL> commit;
Commit complete
SQL> select segment_type,bytes,blocks,extents from user_segments where segment_name like 'RICH%';
SEGMENT_TYPE BYTES BLOCKS EXTENTS
TABLE 3014656 368 23
INDEX 4063232 496 31
SQL> alter index rich_i rebuild;
Index altered
SQL> select segment_type,bytes,blocks,extents from user_segments where segment_name like 'RICH%';
SEGMENT_TYPE BYTES BLOCKS EXTENTS
TABLE 3014656 368 23
INDEX 2752512 336 21
SQL>