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  Sets and Data management

Sets:

A set is a assortment of records. This collection is represented by as well as implemented as an “access path” that runs through the collection of records. Sets carry out the functions of

a) Relating the records of the set.
b) In a few instances directing the physical clustering of records in physical storage.

A record instance may take place in many different sets but it may occur at most once in a particular set.

There are three set kinds of interest:

Sequential set- the records in the set form a solitary sequence. The records in the set are well-organized either by order of arrival (entry sequenced (ES)), by cursor position at insert (CS) or are ordered (ascending or descending) by some subset of field values (key sequenced (KS)). Sequential sets model indexed sequential files (ISAM and VSAM).

Partitioned set- The records in the set type a sequence of disjoint groups of sequential sets. Cursor operators permit one to point at a particular group. Afterwards the sequential set operators are used to navigate within the group. The set is therefore major ordered by hash and minor ordered (ES, CS or KS) within a group. Hashed files in which every group forms a hash bucket are modelled by partitioned sets.

Parent-child set- The records of the set are ordered into a two-level hierarchy. Every record instance is either a parent or a child (but not both). Every child has a unique parent and no children. Every parent has a (possibly null) list of children. Using parent-child sets one is able to build networks as well as hierarchies. Positional operators on parent-child sets comprise the operators to locate parents and operations to navigate on the sequential set of children of a parent. The CONNECT as well as DISCONNECT operators explicitly relate a child to a parent. One acquires implicit connect and disconnect by asserting that records inserted in one set should also be connected to another. (Alike rules apply for delete, connect and update). Parent-child sets can be utilized to support hierarchical as well as network data models.

A partitioned set is a disintegrate form of a parent-child set (the partitions have no parents) as well as a sequential set is a degenerate form of a partitioned set (there is only one partition.) In this discussion be concerned has been taken to define the operators so that they also subset. This has the result that if the program uses the simplest model it will be able to run on any data and as well allows for subset implementations on small computers.

Inserting a record in one set map activate its connection to several other sets. If set “I” is an index for set “F” then an insert, delete as well as update of a record in “F” may trigger a corresponding insert, delete, or update in set “I”. In order to assists this data manager must know:

a) That insertion, deletion or update of a record causes its connection to, movement in or disconnection from other sets.

b) Where to insert the fresh record in the new set:

  • For sequential sets, the ordering should be either key sequenced or entry sequenced.
  • For partitioned sets, data manager should know the partitioning rule and know that the partitions are entry sequenced or key sequenced.
  • For parent-child sets the data manager should know that certain record types are parents and that others are children. Additionally in the case of children data manager should be able to deduce the parent of the child.

We will habitually use the term “file” as a synonym for set.

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