Indexes have keys that maintain a list of matching entries, up to the index entry limit. When that limit is reached, the key will not contain or maintain that list, and will just maintain a count of matching entries. To determine if index keys are approaching their limit, use either the dbtest tool or the verify-index tool.
While the dbtest tool can be used to gather general imformation, the verify-index tool provides statistical data about the percent of entries covered by the keys.
For example, the following command can be used to retrieve a list of keys that have exceeded the entry threshold:
$ bin/verify-index \ --baseDN dc=example,dc=com \ --listKeysExceedingIndexEntryLimit
The following is a sample of the data returned:
[12:06:05] Checked 6003 entries and found 0 error(s) in 2 seconds (average rate 2453.2/sec) [12:06:05] Statistics for records that have exceeded the entry limit: [12:06:05] The st.equality index has 48 such record(s) limit=100 min=103 max=152 median=118 [12:06:05] 1. or (152 entries / 2.53% of all entries) [12:06:05] 2. ma (132 entries / 2.20% of all entries) ..... [12:06:05] The id2subtree index has 2 such record(s) limit=4000 min=6000 max=6002 median=6001 [12:06:05] 1. 1 => dc=example,dc=com (6002 entries / 99.98% of all entries) ..... [12:06:05] The id2children index has 1 such record(s) limit=4000 min=6000 max=6000 median=6000 [12:06:05] 1. 2 => ou=People,dc=example,dc=com (6000 entries / 99.95% of all entries) [12:06:05] The objectClass.equality index has 4 such record(s) limit=4000 min=6001 max=6003 median=6001 [12:06:05] 1. top (6003 entries / 100.00% of all entries) .....