By S. M. Deen (auth.)
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Additional info for Fundamentals of Data Base Systems
Partially Inverted File In a partially inverted file, data records are stored explicitly in addition to an index table for the keys. It is not necessary to represent all the data items as keys. 24 Fundamentals of Data Base Systems Secondary Indexed File This is a modified version of a partially inverted file where one key is selected as primary and the others as secondary. Two index tables, one for the primary keys and the other for the secondary keys, are maintained. The primary indices give the record addresses, whereas the secondary indices refer to the primary keys.
Departure from this involves a complex organisation where a record can be accessed either through multiple keys or through pointers linking the related records. The first of these is the inverted file organisation which permits multiple keys per record. Essentially there are three ways of organising an inverted file, as follows. Fully Inverted File In a fully inverted file, every data item of a record is represented as a key, and an index table is maintained for all their values. Since a data item value could be common to more than one record, the index table consists of entries for every data item value followed by the addresses of all the relevant records.
However, locating a particular entry in the inverted index has its own problems. For example, variable length of the index entry records makes the application of the binary-chops method difficult, although the use of indexed random or indexed sequential techniques can be considered. Updating an entry in the inverted index is another serious problem, since a change in one entry might affect many records. Finally, the reconstruction of records using auxiliary indices in a fully inverted file is very time consuming.