# Columnar format

A file format that stores values grouped by column rather than by row, so a reader can fetch only the fields it needs.

Source: https://university.eugo.io/glossary/columnar-format

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On a wide table this is often the single largest reduction in read volume available, which matters when I/O rather than compute is the bottleneck.

## Related terms

- [I/O-bound](https://university.eugo.io/glossary/io-bound) — A workload whose runtime is limited by reading and writing data rather than by calculation. Adding workers to an I/O-bound job adds waiting, not throughput.
- [Partition](https://university.eugo.io/glossary/partition) — One independently processable chunk of a dataset. Partition size is the main lever on how well a workload parallelizes.