Data warehousing [Wid95] are means to facilitate fast, reliable and less expensive way of collecting and analyzing important information. A data warehouse provides integrated access to the information collected from multiple, distributed and heterogeneous databases. Over the past few years there has been a great interest in this topic in the database industry. Most leading vendors provide some tool for data warehousing. Though there has been high demand for data warehousing tools, most available tools are inflexible and have limited features. These tools are mostly used for decision support queries (often with aggregates), On-line analytical processing (OLAP), and data mining.
OLAP is a technology that enables users to gain insight into data through fast,
consistent and interactive access to a wide variety of possible views of
information. These applications involve dynamic multidimensional analysis of
data. The key requirement of these applications is the ability to aggregate
across many sets of dimensions. Such a computation is a performance bottleneck
for these applications. Aggregate extensions of particular interest in this
context are data cubes and online aggregation.