# Range vs hash partitioning

> Two ways to decide which shard owns a key, and what each does to range queries and hot spots.

Concept · Replication & partitioning · https://levelcode.org/concepts/sharding

Partitioning (sharding) splits data across machines so each stores and serves a slice. A **partition key** decides where each record lives, and the function from key to partition comes in two main forms.

## Range partitioning

Each partition owns a contiguous range of keys: `a–f`, `g–m`, and so on. Used by HBase, Bigtable, CockroachDB, TiDB and MongoDB's ranged sharding.

- **Range scans are efficient:** keys are sorted, so "all events between 10:00 and 10:05" touches one or two partitions.
- **Partitions split and merge** as they grow and shrink, adapting to the data.
- **Sequential keys create hot spots:** with timestamps or auto-increment IDs, every new write goes to the last range.

## Hash partitioning

Hash the key and assign by hash value. Used by Cassandra, DynamoDB and Redis Cluster.

- **Even load:** similar keys land on different partitions, so sequential keys spread out.
- **No range scans across keys:** adjacent keys are scattered; range queries must ask every partition.

## Compound keys: both at once

Hash the first part of the key to pick the partition and sort by the second part within it. In Cassandra, `PRIMARY KEY ((user_id), created_at)` keeps all of a user's rows together, ordered by time — so "a user's latest posts" is one sequential read, while users spread evenly across the cluster.

## Choosing the key

The key matters more than the scheme:

- Pick the field almost every query filters by (`tenant_id`, `user_id`, `conversation_id`), so queries touch one partition.
- Avoid low-cardinality keys (country, status) — you'll have a few enormous partitions.
- Watch for skew: one huge tenant or celebrity makes a [hot partition](https://levelcode.org/concepts/hot-partitions) whatever the scheme.

Never assign hash partitions with `hash(key) mod N`: changing N moves almost every key. Use [consistent hashing](https://levelcode.org/concepts/consistent-hashing) or a fixed number of partitions mapped to nodes. See also [secondary indexes on sharded data](https://levelcode.org/concepts/secondary-indexes-partitioned).