Data warehouse systems use back-end tools and utilities to populate and refresh their data (Figure 3.12). These tools and utilities include the following functions:
Data extraction,which typically gathers data frommultiple, heterogeneous, and external sources
Data cleaning, which detects errors in the data and rectifies them when possible
Data transformation, which converts data from legacy or host format to warehouse format
Load, which sorts, summarizes, consolidates, computes views, checks integrity, and builds indices and partitions
Refresh, which propagates the updates from the data sources to the warehouse