Disclosed is a technology for data storage management in a database system, and more particularly, a data storage technology for a mixed OLAP/OLTP workload. A column group selection apparatus for efficiently storing data in a mixed workload processing environment includes a query processor configured to create column access information about queries that are input, a page monitoring module configured to create page-specific query pattern information using information about a page at which each of the input queries accesses and the column access information, a page layout manager configured to create page column group information in which a column group to be used to form each page is selected by applying a tree-based algorithm for selecting a column group to the page-specific query pattern information, and a data storage manager configured to create and store pages in units of column groups based on the page column group information.