Grain

The grain of a fact table is the precise meaning of a single row — the level of detail it records.

In practice: Declare the grain in business terms before choosing dimensions or facts (“one row per order line”, “one row per daily account balance”). Every dimension and measure on the table must be true at that grain; mixing grains in a single table is the most common dimensional modelling mistake.

Example: fact_sales at the grain one row per product per order line — so quantity and extended_amount are recorded per line, never per whole order.

See also: Standard Cost (declares its grain as an explicit key tuple)