!!! Overview
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|Definition -- ID List] containing the [entry IDs|Definition -- Entry ID] of the entries containing those substrings.  The attribute's substring [matchingRule] is used to [normalize|Definition -- Normalized Value] 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. !! More Information
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