search with indexes

[michel guilhem <michel.guilhem@cp.finances.gouv.fr>]

I try to understand how the indexes operates :
In my slapd.conf i have this line
index default pres,eq,sub,approx

when i launch a ldapsearch with this syntax

ldapsearch -b "o=perso,c=fr" 'ou=29*"

my ldapsearch do a research through the indexes ( my ldap is launched
with debug -d 255 )

but when i do 

ldapsearch -b =o=perso,c=fr" 'ou=2*'

i have a sequential search

I'm looking for a text that gives precisions on indexes , is it possible
to do a search with indexes when i ask for 2* or is it limitations of
indexes ?

thank you very much for the response or a clue