Data engineering·Files, formats and table layers
the query reads three columns out of two hundred and only touches the bytes for those three.
Columnar format
Also calledcolumn-oriented storage, column store file
A file layout that stores all the values of one column together rather than storing rows contiguously. Two things follow, and they are why analytics runs on it: you can skip columns entirely, and values of the same type sitting next to each other compress far better. The cost is on the other side — reading or writing a single whole row touches every column's region, which is why these formats are wrong for transactional workloads.