The Transformation Layer

Concepts: paEltVsEtl, paMedallion, paPartitioning, paColumnarVsRow

The transformation layer is where the interview is won or lost. This is where interviewers spend the most time probing, because it reveals whether you understand data modeling, partitioning, and the ELT vs ETL tradeoff - the single most tested concept in pipeline interviews. ELT vs ETL: The #1 Tested Concept ETL (Extract-Transform-Load) transforms data before loading it into the warehouse. ELT (Extract-Load-Transform) loads raw data first, then transforms it inside the warehouse. This isn't just an acronym difference - it's a fundamental architecture decision that affects cost, flexibility, and debugging. Medallion Architecture (Bronze / Silver / Gold) Medallion architecture organizes your lake into three quality tiers. Bronze is raw, exactly as ingested. Silver is cleaned, deduplicate