The Duplicate-Key Blowup

Concepts covered: paDistributedPrimitives

There is one way a join hurts you that has nothing to do with strategy, and it belongs in the beginner tier because it produces wrong bills and wrong dashboards, not just slow jobs. A join matches every qualifying pair. If a key appears m times on the left and n times on the right, the result contains m times n rows for that key. One-to-one and one-to-many joins behave the way intuition expects. Many-to-many joins multiply, and they usually multiply by accident. Where do the duplicates come from? Almost never from malice. A dimension table that keeps history holds one row per product per version, and joining on product_id alone matches every version. A feed loaded twice inserts every row twice. A join key that is not actually unique, an order_id in a table that is secretly one row per orde

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