What it is: An optional secondary key that, combined with partition key, forms a composite primary key.

What it’s for:

• Query multiple related items within the same partition key (range queries).
• Organize items by time or category (e.g., userId + timestamp).

Key ideas:

• Allows efficient queries like “all orders for user X between time A and B”.

Hard words:

• *composite* /kəmˈpɑːzɪt/: kết hợp
• *range query* /reɪndʒ ˈkwɪri/: truy vấn theo khoảng