The core of data tier is DME which is a part of Oracle database. DME is controlled by ODM API and executes data mining algorithms for the purposes of model creating and model testing. Models are created based on data prepared in the data preparation phase. The models, data about models and data about models testing are stored in the ODM model repository. ODM model repository is controlled by DME and has a predefined structure that can not be altered. As consequence there is a DMDSS model repository which stores other data that DMDSS needs to fully describe the data mining model, e.g. GIF file of decision tree and the purpose of model creation. The union of both repositories represents the full model repository for DMDSS. Data tier is logically divided into three domains. The Oracle DM domain contains DME and ODM model repository. DMDSS domain contains DMDSS model repository and other DMDSS data. Data source domain contains the data that represents a source for data mining algorithms executed by DME.