Index size calculation

Is there a formula to calculate the approximate size of the index for a table like --

(sum of storage bytes of the columns forming index) * number_of_rows

I need to create an index on a large existing table and am trying to figure out the storage parameters.

Thanks,
- Rajeev

select 'ALTER INDEX ' || rtrim(substr(segment_name,1,length(segment_name))) ||
' REBUILD TABLESPACE tablespace_name STORAGE (INITIAL ', trunc(bytes / 1024 / 1024) + 1 || 'M NEXT 1M PCTINCREASE 0) ONLINE ;'
from dba_segments
where extents > 5
and segment_type = 'INDEX'
and owner = user
/

the above sql will work for in 8i.For all other versions pls remove the last word online.

after rebuliding the index do an analyze of the index.

hope this helps.

There is an excellent section in Appendix A of the Oracle8 Administrator's Guide that discusses appropriately estimating space requirements for tables, indexes and clusters.

Joe Ramsey