The Access Methods Manager provides an abstraction of permanent data in terms of collections of records stored in a heap file and indexes which consists of a collection of records of the form <key value,RID>, where key value is a value for the search key of the index, and RID is the identifier of a record in the relation being indexed. Any number of indexes can be defined on a relation, and a search key can be multiattributes. The operators provided by the Access Methods Manager are used to implement the physical operators of access plans generated by the query optimizer.
Records of an heap file or an index are accessed by scans. An heap file scan operator simply reads each records one after the other, while an index scan allows to retrieve efficiently the RID of the records with a search key values in a given range.