Fatma Ozcan

Multidatabase Query Optimization

A multidatabase system (MDBS) allows the users to simultaneously access heterogeneous,and autonomous databases using an integrated schema and a single global query language. The query optimization problem in MDBSs is quite different from the query optimization problem in distributed homogeneous databases due to schema heterogeneity and autonomy of local database systems. In this work, we consider the optimization of query distribution in case of data replication and the optimization of intersite joins, that is, the join of the results returned by the local sitesin response to the global subqueries. The algorithms presented for the optimization of intersite joins try to maximize the parallelism in execution and take the federated nature of the problem into account. It has also been shown through a comparativeperformance study that the proposed intersite join optimization algorithms are efficient.The approach presented can easily be generalized to any operation required for intersite query processing.The query optimization scheme presentedin this paper is being implemented within the scopeof a multidatabase system which is based on OMG‘sobject management architecture.

METU interoperable database system

METU INteroperable Database System (MIND) is a multidatabase system that aims at achieving interoperability among heterogeneous, federated DBMSs. MIND architecture if based on OMG distributed object management model. It is implemented on top of a CORBA compliant ORB, namely, ObjectBroker. MIND provides users a single ODMG-93 compliant common data model, and a single global query language based on SQL. This makes it possible to incorporate both relational and object oriented databases into the system. Currently Oracle 7, Sybase and METU OODBMS (MOOD) have been incorporated into MIND. The main components of MIND are a global query processor, a global transaction manager, a schema integrator, interfaces to supported database systems and a user graphical interface.In MIND all local databases are encapsulated in a generic database object with a well defined single interface. This approach hides the differences between local databases from the rest of the system. The integration of export schemas is currently performed manually by using an object definition language (ODL) which is based on OMGs interface definition language. The DBA builds the integrated schema as a view over export schemas. the functionalities of ODL allow selection and restructuring of schema elements from existing local schemas.MIND global query optimizer aims at maximizing the parallel execution of the intersite joins of the global subqueries. Through MIND global transaction manager, the serializable execution of the global transactions are provided.

A multidatabase system implementation on CORBA

METU Interoperable DBMS (MIND) is a multidatabase system based on OMGs distributed object management architecture. It is implemented on top of a CORBA compliant ORB, namely, DECs ObjectBroker. In MIND, all local databases are encapsulated in a generic database object. The interface of the generic database object is defined in CORBA IDL and multiple implementations of this interface, one for each component DBMS, namely Sybase, Adabas D and MOOD are provided. MIND provides its users with a common data model and a single global query language based on SQL. The main components of MIND are a global query manager, a global transaction manager, a schema integrator, interfaces to supported database systems and a graphical user interface. The integration of export schemas is currently performed by using an object definition language (ODL) which is based on OMGs interface definition language. MIND global query optimizer aims at maximizing the parallel execution of the intersite operations of the global subqueries. Through the MIND global transaction manager, the serializable execution of the global transactions (both nested and flat) is provided.

Dynamic query optimization on a distributed object management platform

A Distributed Object Management (DOM) architecture, when used as the infrastructure of a multidatabase system, not only enables easy and flexible interoperation of D13MSS, but also facilitates interoperation of the multidatabase system with other repositories that do not have DIBMS capabilities. Thk is an important advantage, since most of data still resides on repositories that do not have DIBMS capabilities. In thk paper, we describe a dynamic query optimization technique for a multidatabaae system, namely MIND, implement ed on a DOM environment. Dynamic query optimization, which schedules intersite operations at runtime, fits better to such an environment since it benefits from location transparency provided by the DOM framework. In thk way, the dynamic changes in the configuration of system resources such as a relocated DBMS or a new mirror to an existing DBMS, do not affect the optimized query execution in the system. Furthermore, the uncertainty in estimating the appearance times (i.e., the execution time of the global subquery at a local DBMS) of partial results are avoided because there is no need for the dynamic optimizer to ‘know the logical cost parameters of the underlying local DBMS. In scheduling the intersite operations a statistical decision mechanism is used. We also consider the schema inte~ation information to make room for further query optimizak ion. For this purpose, a method is presented that forms global query graphs by taking the schema integration information into account which is then used by the dynamic query optimizer. The proposed scheme tries to exploit the inherent parallelism in the system as much as possible. The performance of the developed method is compared with two other most related techniques and the results clf the * This work is partially being supported by the Turkish State PIanning Organization, Project Number: AF P-0312 DPT.95K120500, by the Scientific and Technical Research Council of Turkey, Project Number: EEEAG-Yazilim5, by Motorola (USA) and by Sevgi Holding (’Ihrkey) Permission to make digitsVbard copies of nli or part of thk material for personal or class~m use is granted without fee provided that the copies are not made or dwtributed for profit or commercial advantage, the copyright notice, the title of the publication and its date appear, and notice is given that copyright is by permission of rbe ACM, Inc. To copy otherwise, to republish, to poston serversor to redistributeto lists, requireaspecific permissionand/or fee. CIKM 96, Rockville MD USA @ 199(j ACM @89791 +73+/9fj/l 1 ..

Dynamic Query Optimization in Multidatabases.

3 .5~ 117 experiments indicate that the dynamic query optimization technique presented in thk paper has a better performance.