The design of a canonical database system (PRECI)

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Abstract
PRECI is based on a canonical data model potentially capable of supporting user views of other models – notably CODASYL and relational ones – through local schemas and appropriate data manipulation languages. The canonical global schema consists of normalised relations and is backed up by a storage and a data dictionary. The model is being implemented at Aberdeen University as a generalised database system, to be used primarily as a tool for research in databases, with a modular design approach so that future changes can be incorporated easily. The CODASYL and relational subschema facilities are currently being developed; a relational algebra to be used for DM commands from FORTRAN programs has been provided. The storage and access strategy in PRECI is based on internal record identifiers (or surrogates) created largely in entity-identifier order. Entity records can be accessed very quickly – both randomly and sequentially – by surrogates or entity identifiers, partly with the help of a novel indexing technique, called hash tree, which is based on data compression and hashing. The model provides maximal data independence through load-time and run-time binding. The local user may define access paths independently of the storage schema. Data items, record types and set types can be added to or deleted from the global schema; the storage schema can be restructured and data pages and indexes can be easily reorganised. Changes in one schema do not require recompilations of the unaffected schemas. The DBA has complete control over the storage and indexing strategy which he can manipulate to improve performance. Five levels of optimisation are provided to enhance execution efficiency and minimise memory usage. Certain integrity checks are also carried out during run-time.

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