- what are you indexing? 10M rows with PDF documents or just simple plain text?
- what is your hardware?
- how is your index made-up? Is it just a plain index or do you use all kind of features (substring etc)?
- Which version of Oracle are you using?
In the oracle documentation you can find information about this issue: http://docs.oracle.com/cd/E11882_01/text.112/e24435/aoptim.htm#i1006756