A substring index is a type of LDAP Indexes that is used to keep track of which entries contain specific substrings.
Index keys for a substring index consist of six-character substrings taken from attribute values and the corresponding values are ID lists containing the entry IDs of the entries containing those substrings. The attribute's substring matchingRule is used to normalize the values for the index keys, and substring indexes cannot be defined for attributes that do not contain substring matching rules.
Substring matches a subset of the attribute value string. For example, a query to find a LastName with “der” would return matches for Derington, Anderson, and Lauder.
Substring indexes is the most resource-intensive index to create and maintain.