ETL Loading process

In loading stage, some data are loaded to the target directly without applying any transformation logic and some data are loaded to the target after applying the logic or business rules. The load phase loads the data into the end target, usually the Data Warehouse (DW). Depending on the requirements of the organization, this process varies widely. Some data warehouses may overwrite existing information with cumulative, updated data every week, while other DW (or even other parts of the same DW) may add new data in a historized form, for example, hourly. The timing and scope to replace or append are strategic design choices dependent on the time available and the business needs. More complex systems maintain a history and audit trail of all  changes to the data loaded in the DW

As the load phase interacts with a database, the constraints defined in the database schema, as well as in triggers activated upon data load apply (for example, uniqueness, referential integrity, mandatory fields), which also contribute to the overall data quality performance of the ETL  process.