Partition Keys and Sort Keys

Choosing the right primary key is the single most important design decision in DynamoDB. This lesson explains how partition keys and sort keys work, how DynamoDB distributes data, and how to design keys that enable efficient access patterns.

---

How DynamoDB Stores Data

DynamoDB uses partitioning to distribute data across multiple storage nodes:

1. When you write an item, DynamoDB hashes the partition key value
2. The hash determines which physical partition stores the item
3. Within a partition, items are sorted by the sort key (if one exists)

                     Hash Function
                         │
        ┌────────────────┼────────────────┐
        ▼                ▼                ▼
   Partition A      Partition B      Partition C
   ┌──────────┐    ┌──────────┐    ┌──────────┐
   │ Items     │    │ Items     │    │ Items     │
   │ sorted by │    │ sorted by │    │ sorted by │
   │ sort key  │    │ sort key  │    │ sort key  │
   └──────────┘    └──────────┘    └──────────┘

Each partition can store up to 10 GB of data and handle 3,000 read capacity units (RCUs) and 1,000 write capacity units (WCUs).

---

The Partition Key

The partition key (also called the hash key) determines data distribution:

Good Partition Keys

A good partition key has high cardinality — many distinct values that spread data evenly:

Table	Good Partition Key	Why
Users	UserId	Unique per user
Orders	OrderId	Unique per order
IoT Readings	DeviceId	Many devices, even distribution
Sessions	SessionId	Unique per session (UUID)

Bad Partition Keys

A bad partition key creates hot partitions — too many items on a single partition:

Table	Bad Partition Key	Why
Orders	Status	Only a few values (Pending, Shipped, Delivered)
Events	Date	All today's events go to one partition
Users	Country	Uneven — most users may be in one country
Logs	LogLevel	Only ERROR, WARN, INFO, DEBUG

Tip: If your natural key has low cardinality, consider adding a suffix (e.g., Date#ShardId) to spread the load.

---

The Sort Key

The sort key (also called the range key) enables rich queries within a partition:

Sort Key Query Operators

Operator	Description	Example
=	Exact match	OrderDate = "2024-01-15"
<, <=, >, >=	Comparison	OrderDate > "2024-01-01"
BETWEEN	Range	OrderDate BETWEEN "2024-01-01" AND "2024-03-31"
begins_with	Prefix match	SK begins_with "ORDER#"

Hierarchical Sort Keys

You can encode hierarchies in sort keys using delimiters:

Table: Locations
Partition Key: Country
Sort Key: Region#City#Postcode

┌──────────┬───────────────────────────────┐
│ Country  │ Sort Key                      │
├──────────┼───────────────────────────────┤
│ UK       │ England#London#SW1A           │
│ UK       │ England#London#EC1A           │
│ UK       │ England#Manchester#M1         │
│ UK       │ Scotland#Edinburgh#EH1        │
│ UK       │ Wales#Cardiff#CF10            │
└──────────┴───────────────────────────────┘