Glossary

Grain

The grain of a fact table is the precise business definition of what a single row represents: one row per order line, one row per account per month, one row per boarding-pass scan. It is declared first in dimensional design — before choosing dimensions or measures — because it is the test everything else must pass: a dimension belongs only if it’s single-valued at the grain; a measure belongs only if it’s true at the grain.

Mixing two grains in one table is the classic dimensional modeling bug, and the root cause of most double-counting. The standard advice is to declare the most atomic grain the source produces: detail can always be aggregated up, but a coarse grain can never be drilled back down.

Go deeper: The Grain of a Fact Table: The First Decision That Decides Everything Else