Assessment of impact of selected factors on the effectiveness of cost-based optimizer in database systems

The paper presents the results of experiments, which aim was to evaluate the influence of selected factors on the cost and time of the query in database systems. The cost and execution time are criteria of assessment of query plans by the cost-based optimizer of a database system. Essential for constructing queries and for the efficiency of cost-based optimizer are: indexing, aliasing, asymmetry of the data and the availability of statistics referring to the tables. Experiments confirmed that the appropriate way to implement a query, based on the use of indexation or aliasing, and on modification of query parameters with the systematic generation of statistical information about the data can significantly reduce the time to obtain query results, offload the processor and reduce the number of disk reads. The experiment was conducted using the real data in the database ORACLE