Question on slapd.conf indexes and attribute properties in schemas

[Tak Fujiwara <tak.fujiwara.linux@gmail.com>]

First Question:
What is the purpose of the "pres" index?  How can I use it when I
search for particular entries?

This is taken from the LDAP Admin manual to cite an example:

index default pres,eq
index uid
index cn,sn pres,eq,sub
index objectClass eq

The first line sets the default set of indexes to maintain to present
and equality. The second line causes the default (pres,eq) set of
indices to be maintained for the uid attribute type. The third line
causes present, equality and substring indices to be maintained for cn
and sn attribute types. The fourth line causes an equality index for
the objectClass attribute type.

Second Question:
How do these indexes work?  Like for instance, in the schema, you can
use substrings for mail.  I can use ldapsearch to search for an email
and find the entry successfully using substrings without having to
create a "mail sub" entry for mail in slapd.conf.  However, searching
for a specfic given name using ldapsearch and setting a "gn eq" in
slapd.conf would yield no results.

Tak