The usual databases store their data record by record. This has advantages for transactional databases if one is not interested in the correlation between records. For typical queries in Business Intelligence however thousands of records are analyzed together. Storing the columns in a column-based fashion, often combined with compression techniques, often gives large performance benefits. These databases are called "column-oriented DBMS".
Here are some open source products.
There are a few frameworks, components and tools for Business Intelligence under an open source license.
The following web sites provide general information.