Explain Partition Key vs Sort Key.

A partition key determines which partition stores data, while a sort key defines the order of items within that partition.

When to Use

Use a partition key when you need even data distribution across servers. Add a sort key when you want multiple related items stored together and retrieved in order—like a user’s posts sorted by time.

Example

In an Orders table, CustomerID can be the partition key, grouping all orders by customer, while OrderDate can be the sort key to sort orders chronologically.

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Why Is It Important

Choosing the right partition and sort keys ensures scalability, avoids hotspots, and makes queries efficient—critical in systems like DynamoDB.

Interview Tips

In interviews, explain clearly: partition key = distribution, sort key = ordering. Use a quick example (e.g., UserID + Timestamp). Mention trade-offs and query restrictions to stand out.

Trade-offs

Partition keys scale data well, but without a sort key, related queries are harder. Sort keys allow grouping but can create “hot” partitions if unevenly used.

Pitfalls

Common mistakes include picking a partition key that causes uneven distribution or assuming you can query solely by sort key—DynamoDB requires the partition key.