The presence index includes all entries in the database that have a value for a specified attribute, irrespective of that value. The following figure shows a presence index for the nsRoleDN attribute. For information about this attribute, see nsRoleDN(5dsat).
Directory Server uses the value of the entryid attribute to store a reference to the entry. Directory Server retrieves the entry by using the instance-path/db/dbinstance/dbinstance_id2entry.db3 index file, where dbinstance depends on the database identifier.
When Directory Server receives a request to remove an attribute value indexed for presence, it must remove the entry from the presence index for that attribute before acknowledging the update to the client application.
The cost of presence indexes is generally low, although the list of entries maintained for a presence index may be long. When the index list length is small, presence indexes are useful for attributes in a relatively small percentage of directory entries.