Data-Warehouse on Ivan Luminaria

Ragged hierarchies: when the client has no parent and the group has no grandparent

Tue, 20 Jan 2026 08:03:00 +0100

Three levels. Top Group, Group, Client. It looks like a trivial structure — the kind of hierarchy you draw on a whiteboard in five minutes and that any BI tool should handle without issues.

Then you discover that not all clients belong to a group. And that not all groups belong to a top group. And that the aggregation reports the business asks for — revenue by top group, client count by group, drill-down from the top to the leaf — produce wrong or incomplete results because the hierarchy has holes.

SCD Type 2: the history the business didn't know it needed

Tue, 11 Nov 2025 08:03:00 +0100

The sales director shows up at the Monday morning meeting with a simple question: “How many customers did we have in the North region last June?”

The DWH’s answer: silence.

Not because the system was down, or the table was missing. The data was there, technically. But it was wrong. The DWH returned the customers currently in the North region — not the ones that were there in June. Because every night, the loading process overwrote the customer master data with current values, erasing any trace of what came before.

Wrong grain: when the fact table can't answer the right questions

Tue, 21 Oct 2025 08:03:00 +0100

The meeting had started well. The sales director of an industrial distribution company — around sixty million in revenue, three thousand active customers, a catalog of twelve thousand SKUs — had opened the new data warehouse presentation with a smile. The numbers matched, the dashboards were polished, the monthly totals by agent and territory reconciled with accounting.

Then someone asked the wrong question. Or rather, the right one.

“Can I see what customer Bianchi purchased in March, line by line, product by product?”