Why Index size is bigger than table size?

Question

Dear All,
I found in my database my tables sizes is coming around 30TB (All Tables in Database). and my index size for the same is 60TB. This is data ware housing environment.
How the index size and table size are differing?
Why they are differing? why index size is bigger than table size?
How to manage the size?
Please give me clear explanation and required information on the above.
Regards
Suresh

Answer 1

There are many reasons why the total space allocated indexes could be larger than the total space allocated to tables. Sometimes it's a mark of good design, sometimes it indicates a problem. In your position your first move is to spend as little time as possible in deciding whether your high-level summary is indicative of a problem, so you need to look at a little more detail.
As someone else pointed out - are you looking at the sizes because you are running out of space, or because you have a perceived performance problem. If not, then your question is one of curiosity.
If it's about performance then you should be looking for code (either through statspack/AWR or sql_trace) that is performing badly and use the analysis of that code to help you identify suspect indexes.
If it's about space, then you need to do some simple investigations aimed at finding a few indexes that can be "shrunk" or dropped. Pointers for this are:
select
        table_owner, table_name, count(*)
from
        dba_indexes
group by
        table_owner, table_name
having
        count(*) > 2   -- adjust to keep the output short
order by
        count(*) desc;This tells you which tables have the most indexes - check the sizes of the tables and indexes and then check the index definitions for the larger tables with lots of indexes.
Second quick check - join dba_tables to dba_indexes by table_name, and report the table blocks and index leaf blocks in desending order of leaf block count. Look for indexes which are very big, and also bigger than their underlying tables. There are special cases (and bugs) that can cause indexes to be much bigger than they need to be ... this report may identify a couple of anomalies that could benefit from an emergency fix followed (possibly) by a strategic fix.
Regards
Jonathan Lewis