The equality index includes all entries in the database that have a specified value for a given attribute. This index requires a value to be specified in the search filter. The following figure shows an equality index for the sn, surname, attribute. The index maintains a list of values for the sn attribute. For information about this attribute, see sn(5dsat).
When Directory Server receives a request to update an entry indexed for equality, it must do the following tasks before performing the update and acknowledging the update to the client:
Determine whether the entry must be removed from the index
Determine whether a list must be added to or removed from the index
The cost of equality indexes is generally lower than for substring indexes, but equality indexes require more space than presence indexes. Some client applications such as messaging servers might rely on equality indexes for search performance. Avoid using equality indexes for large binary attributes such as photos and hashed passwords.