Editing IDMS (section)

===Logical Data Model===
The data model offered to users is the [[CODASYL]] network model. The main structuring concepts in this model are records and sets. '''Records''' essentially follow the [[COBOL]] pattern, consisting of fields of different types: this allows complex internal structure such as repeating items and repeating groups.

The most distinctive structuring concept in the Codasyl model is the '''set'''. Not to be confused with a mathematical set, a Codasyl set represents a one-to-many relationship between records: one owner, many members. The fact that a record can be a member in many different sets is the key factor that distinguishes the network model from the earlier [[hierarchical model]]. As with records, each set belongs to a named set type (different set types model different logical relationships). Sets are in fact ordered, and the sequence of records in a set can be used to convey information. A record can participate as an owner and member of any number of sets.

Records have identity, the identity being represented by a value known as a '''database key'''. In IDMS, as in most other Codasyl implementations, the database key is directly related to the physical address of the record on disk. Database keys are also used as pointers to implement sets in the form of linked lists and trees. This close correspondence between the logical model and the physical implementation (which is not a strictly necessary part of the Codasyl model, but was a characteristic of all successful implementations) is responsible for the efficiency of database retrieval, but also makes operations such as database loading and restructuring rather expensive.

Records can be accessed directly by database key, by following set relationships, or by direct access using key values. Initially the only direct access was through hashing, a mechanism known in the Codasyl model as CALC access. In IDMS, CALC access is implemented through an internal set, linking all records that share the same hash value to an owner record that occupies the first few bytes of every disk page.

In subsequent years, some versions of IDMS added the ability to access records using [[Btree|BTree]]-like indexes.