1. A system comprising: a first virtual table corresponding to a first data; a second virtual table corresponding to a second data; a common table that is associated with the first and second virtual tables; a list with information about associations between the first and second virtual tables and the common table; a generating section for generating two or more access plans from a database query on the common table, the two or more access plans involving at least one of the first and second virtual tables in accordance with the list; and a selecting section for selecting one of the two or more access plans generated by the generating section to be used to execute the database query.