By D. A. Bell
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Extra info for Relational Databases. State of the Art Report
Sample text
Consider a semi-hypothetical database as illustrated by the logical data model in Figure 2 which consists of eight relations. A function to be applied to this database is specified in English as follows: Function 1: determine from the current project status which stock needs to be reordered and place an order for it. 29 R1 R2 R3 R4 R5 R6 R7 R8 Outstanding orders Product Parts-materials Work-in-progress PERT-network Process-sheet Modifications Work-centre (£) V y (R6JL I TR5J* 5000 tuples 3000 tuples 20 000 tuples 400 tuples 160 000 tuples 4000 tuples 100 tuples 200 tuples iß 1 :n TRI) ^ Θ w ^^ m:n "^ ^^ Figure 2: Example data model This function accesses five of the relations in Figure 2, namely: Work-in-progress (R4), PERT-network (R5), Process-sheet (R6), Parts-materials (R3) and Outstanding orders (RI).
The method can be used in conjunction with a placement method (for example BEL1) to enhance the access performance for the competing predictable and important queries, while the ad hoc or other non-predicted queries may be optimised by query decomposition. There is considerable scope for improvement of this method. For example, including capture of information on hit groups of prominent sets of tuples in specific relations, tailoring the method of AR calculation to local needs, and using 'scenarios' such as those used in EOS (BEL6), can all help to eliminate some of the work of the clustering and placement algorithms.
The relational catalogue An integral component of SQL/DS is the catalogue, in which all objects defined within the system are automatically recorded. The creation or dropping of tables, views and indexes, for example, the definition of user IDs and granting of access authorisation all result in the updating of the appropriate entries in the catalogue. The catalogue also contains data of an operational nature, such as the status of DBSPACEs and statistics on table contents. The catalogue is itself a collection of SQL/DS tables.