The Directory Server uses indexes to improve database search performance and provide consistent search rates regardless of the number of database objects stored in the DIT. You can specify an index entry limit property, which defines the maximum number of entries that are allowed to match a given index key before it is no longer maintained by the server. If the index keys have reached this limit (which is 4000 by default), then you must rebuild the indexes using the rebuild-index tool as follows:
$ bin/rebuild-index --baseDN dc=example,dc=com --index objectclass
In the majority of Directory Server environments, the default index entry limit value of 4000 entries should be sufficient. However, group-related processing, it may be necessary to increase the index entry limit. For directories containing more than 4000 groups with the same structural object class (i.e., more than 4000 entries, 4000 groupOfUniqueNames entries, 4000 groupOfEntries entries, or 4000 groupOfURLs entries), then you may want to increase the index entry limit for the objectClass attribute so that it has a value larger than the maximum number of group entries of each type. Set index-entry-limit property using a command line like the following:
$ bin/dsconfig set-local-db-index-prop --backend-name userRoot \
  --index-name objectClass --set index-entry-limit:175000

As an alternative, a separate backend may be created to hold these group entries, so that an unindexed search in that backend yields primarily group entries. If you make no changes, then the internal search performed at startup to identify all groups and any user searches looking for groups of a given type may be very expensive.

For directories in which any single user may be a member of more than 4000 static groups of the same type, you may need to increase the index entry limit for the member and/or uniqueMember attribute to a value larger than the maximum number of groups in which any user is a member. If you do not increase the limit, then searches to retrieve the set of all static groups in which the user is a member may be unindexed and therefore very expensive.