Comparison and Implementation of Query Containment Algorithms for XPath

Abstract: This thesis investigates the practical aspects of implementing Query Containment algorithms for the query language XPath. Query Containment is the problem to decide if the results of one query are a subset of the results of another query for any database. Query Containment algorithms can be used for the purpose of optimising the querying process in database systems. Two algorithms have been implemented and compared, The Canonical Model and The Homomorphism Technique. The algorithms have been compared with respect to speed, ease of implementation, accuracy and usability in database systems. Benchmark tests were developed to measure the execution times of the algorithms on a specific set of queries. A simple database system was developed to investigate the performance gain of using the algorithms. It was concluded that The Homomorphism Technique outperforms The Canonical Model in every test case with respect to speed. The Canonical Model is however more accurate than The Homomorphism Technique. Both algorithms were easy to implement, but The Homomorphism Technique was easier. In the database system, there was performance to be gained by using Query Containment algorithms for a certain type of queries, but in most cases there was a performance loss. A database system that utilises Query Containment algorithms for optimisation would for every issued query have to evaluate if such an algorithm should be used.