Complex applications, such as design appl_ications, multi-media applications, and even advanced buszness applications can benefit significantly from a database language that supports composite (or complex) Motivation and IntroductionExisting "second generation" data base systems are focussed on business applications (e.g. booking, storekeeping, cost accounting, project management and decision support). It is widely agreed now that second generation DBMSs are inadequate for a broader class of applications that deal with composite objec~ (s~ct~ed data), as opposed t~ flat rel~ tions. Such apphcauons mclude office automauon, medicine, computer aided software engineering (C_AS~). artificial intelligence (AI), hypertext, and geographical mformation systems and computer aided design (CAD).Many of these applications extensively navigate through the data. Some applications, such as CAD, process the data by complex and time consuming algorithms, and demand very high performance. For this, the data needs to be represented by data structures which ~an. be a~ccss~ very f~st. Caching of data close to the apphcauons 1s parucularly Im- portant when applications/tools. run on autonomous. w~rk stations, with remote access to mtegrated data repositones. For example, design applications deal with large af!lount of data, which is mostly organized using such !llodeling concepts as version, alternative, and co~figurauon. Those applications often work on a well-specified set of data, called working set, s~h as a particul~ version of a documen_t or a wing of an arrcraft for a particular model and vers10n. Such data is typically an aggregation of a set of othe~ documents/designs with specified versions. Usu~ly wor~mg sets are extracted from the database and loaded mto mam memory close to the applications for high performa~ce. After an application completes its work on the wo~k_ing. set, the DBMS propagates back the changes to the ongmatmg databases. Working sets are typically much smaller than the whole database. For example, in design applications, the sizes of the databases are in the gigabytes to terabytes range, whereas working sets are typically. in the range of 1 ~o 100 megabytes. Thus, loading a workmg set translates mto a data extraction where on average one tuple out of 10000 to 100000 is selected. This again calls for set-oriented query facilities for efficient data extraction, requiring powerful optimization, and high performance execution. !fie ~oncept of views is extensively used in DBMSs, allo~mg_ different applications to view the data differently. Apphcauons dealing with composite objects require this concept to be extended to allow the formation of different views over the same data. Further,these views must be updatable.We propose an extension of SQL, called SQL Exten~ed Normal Form (or XNF) to satisfy the needs of the applications discussed above. XNF is based on the concept of Composite Objects (<;0) as a coll~tion of tabl~s and rela~on ships· COs prov1de an abstractiOn mechamsm that umfies both the struct...

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-We present a novel approach for supporting Composite Objects (CO) as an abstraction over the relational data. This approach brings the advanced CO model to existing relational databases and applications, without requiring an expensive migration to other DBMSs which support CO. The concept of views in relational DBMSs (RDBMS) gives the basis for providing the CO abstraction. This model is strictly an extension to the relational model, and it is fully upward compatible with it. We present an overview of the data model. We put emphasis in this paper on showing how we have made the extensions to the architecture and implementation of an RDBMS (Starburst) to support this model. We show that such a major extension to the data model is in fact quite attractive both in terms of implementation cost and query performance. We introduce a CO cache for navigation through components of a CO. With this technique, the performance of navigation through COs, which has been of a concern in RDBMSs in the past, is in fact quite satisfactory. We present our practical experience in using this facility. We show that our work on CO enables existing RDBMSs to incorporate efficient CO facilities at a low cost and at a high degree of application reusability and database sharability. MOTIVATIONIt is widely agreed now that complex applications, such as design applications, multi-media and AI applications, and even advanced business applications can benefit significantly from database interfaces that support composite (or complex) objects (shortly, CO) [15,23]. A generally accepted characterization defines a CO consisting of several components (possibly from different types) with relationships in between [2, 5, 11, 26, 36, 19}. Interestingly, object oriented DBMSs (OODBMSs) have adopted a very similar model [1,43,14,27]. This is particularly true for OODBMSs used in practice. Especially 00 programming environments have made advances in handling of COs.Such environments facilitate the growth of complex applications. As a result, there is considerable pressure on RDBMSs for better support of COs. To respond to this demand, several systems today are bridging 00 environments with relational. An example of such a system is the Persistence DBMS [20], which builds a layer on the top of RDBMSs, providing better support for COs. These systems essentially extract the data from the relational and port data to 00 environments. · One straightforward way of extracting data with complex structure is to follow the parent/child relationships: for each parent instance, execute a query to get the children; repeat the same thing for each child to get its children, and so on. However, this style of data extraction leads to numerous queries, and does not lend itself to effective set-oriented processing. Essentially, the process of data extraction is broken into fragmented queries where the number of fragments is in the order of number of instances of parent components in the extracted data. A better approach is to employ much more powerful set-oriented p...

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