RDB$DB_KEY lets you retrieve a specific record from the database
    using a logical pointer called a database key (dbkey). A dbkey
    indicates a specific record in the database. You can retrieve
    this key as though it were a field in the record. Once you
    have retrieved the database key, you can use it to retrieve
    its associated record directly, as part of a record selection
    expression. The database key gives you the ability to keep
    track of a subset of records in the database and retrieve them
    repeatedly, without using data manipulation syntax.