ETL arose to solve a problem of providing businesses with clean and ready-to-analyze data. We remove dirty and irrelevant data and transform, enrich, and reshape the rest. The example of this could be sessionization: the process of creating sessions out of raw pageviews and users’ events.
ETL is complicated, especially the transformation part. It requires at least several months for a small-sized (less than 500 employees) company to get up and running. Once you have the initial transform jobs implemented, never-ending changes and updates will begin because data always evolves with business.
The other problem of ETL is that during the transformation, we reshape data into some specific form. This form usually lacks some data’s resolution and does not include data that is useless for that time or for that particular task. Often, “useless” data becomes “useful.” For example, if business users request daily data instead of weekly, then you will have to fix your transformation process, reshape data, and reload it. That would take a few weeks more.
Read on for more, including his argument for why ELT is better.