This paper is a research of problem which was faced during development of clinical information system. This is a scope, where one clinical test can by characterised by several hundreds of attributes. Thus the focus is on the design and performance of database tables with 200-300 columns. To perform the research the Oracle Database Management System was chosen. There is the main factors, influencing performance, such as principles of data storage on disk, query processing also the main facilities to improve performance observed. Based on the basic principles of relational database design some alternatives to tables with many attributes were created. It is normalized and denormalized structures. There is performed the experiment to see how the speed of most used queries depends on data structure. Also there is done the analysis of complexity and usability of these queries.