Currently, the majority of the existing IT systems and those that are under development are based on applications that work with database servers. The ever wider range of functionalities that such servers are able to provide results in an increase in the number of their users and in the amount of data processing. This leads to performance problems. Such problems are addressed through both programming and hardware solutions. In terms of programmatic solutions the authors' interest is focused on table structures in RDBMS's and their performance in terms of write operations on data. So far analyses have been conducted relating to the impact of the presence of a primary key and the way it is generated on data write operations in a sample of relational databases.