Huge ecommerce website: show sales rank by category

Medium e-commerce: Use SQL normalized tables. Index by product id & category.

Huge: Move to NoSQL. Composite index (partition key & sort key) by product id & category. Or secondary indexes. Or denormalized.

In CA, AP, CP, choose AP: eg: Cassandra or DynamoDB.

For lower cost, use Hadoop with indexes on product id & category.