OpenDJ Administration Guide
8.2. Configuring & Rebuilding Indexes

As the number of entries in your directory grows, it can make sense not to maintain indexes for particular values. For example, every entry in the directory has the value top for the objectClass attribute, so maintaining a list of entries that match the filter (objectClass=top) is not a reasonable use of resources. In a very, very large directory, the same can be true for (givenName=John) and (sn=Smith).

In an index, each index key points to a list of entries that are candidates to match. For the objectClass index key that corresponds to =top, the list of entries can include every entry in the directory.

OpenDJ directory server therefore defines an index entry limit. When the number of entries that an index key points to exceeds the index entry limit, OpenDJ stops maintaining the list of entries for that index key.

The default index entry limit value is 4000. 4000 is usually plenty large for all index keys, except for objectClass indexes. If you have clients performing searches with filters such as (objectClass=person), you might suggest that they adjust the search to be more specific, such as (&(mail=username@maildomain.net)(objectClass=person)), so that the server can use an index, in this case equality for mail, to limit the number of candidate entries to check for matches.

You can change the index entry limit on a per index basis.

$ dsconfig
 set-local-db-index-prop
 --port 4444
 --hostname opendj.example.com
 --bindDN "cn=Directory Manager"
 --bindPassword password
 --backend-name userRoot
 --index-name objectClass
 --set index-entry-limit:5000
 --trustAll
 --no-prompt
$ rebuild-index
 --port 4444
 --hostname opendj.example.com
 --bindDN "cn=Directory Manager"
 --bindPassword password
 --baseDN dc=example,dc=com
 --index objectclass
 --start 0
Rebuild Index task 20110607160349596 scheduled to start Jun 7, 2011 4:03:49 PM

Alternatively, you can configure the index entry limit for all indexes stored in a backend by using the dsconfig set-backend-prop command with the --backend-name backendName --set index-entry-limit:limitValue options.