Some index entries become so large as to be useless. For example, if every entry in your database is a person entry, the ``=PERSON'' index entry in the objectclass index contains every entry. This returns very little useful information, and can cause significant delays, especially on updates. To alleviate this problem, the package supports the idea of an allIDs index entry.
The allIDs entry stands for a real index entry containing the IDs of every entry in the database, but it takes up very little space, never needs updating, and can be manipulated quickly and efficiently. The trade-off is that it does not prune the set of candidate entries at all during a search. This must be done using other, more ``high-powered'' index entries.
You can set the minimum number of IDs that an index entry may contain before it turns into an allIDs block. The actual number, which is specified using the configuration manager (see ``Configuring an LDBM database''), is determined at runtime by the LDBM backend, depending on the block size of the underlying device (that is, the number you provide is rounded up to the nearest multiple of a block size).