Bytes Scanned (Query Data Volume)
ConceptBytes scanned is the physical volume of data a database or cloud warehouse reads from storage to execute a query, determining cost and speed.
Imagine asking a librarian: "How many customers bought red shoes?" If the librarian opens a 1,000-page book and reads every single page from cover to cover just to find three sentences, the librarian had to "scan" all 1,000 pages. In modern databases, you pay for how many pages the database had to read, not how short the final answer was.
A database table contains 50 Gigabytes of user activity. Running `SELECT *` reads all 50 GB. Selecting only the `order_id` and `amount` columns reads only 2 GB for the exact same business answer.
Running `SELECT *` on wide tables with 50+ columns is the #1 cause of slow queries and surprise cloud bills. Modern analytical databases are columnar — selecting only the 2 columns you need reads 95% less data.
Last reviewed August 28, 2026.