What it is: A secondary index that uses the same partition key as the base table but a different sort key.

What it’s for:

• Query different sort orders within the same partition key.

Key ideas:

• Must be created at table creation time (typical exam point).
• Useful when you know the access pattern early.

Exam cues:

• “need same partition key but alternate sort key” → LSI.

Hard words:

• *local* /ˈloʊkəl/: cục bộ (cùng partition key)
• *created at table creation* /kriˈeɪtɪd æt ˈteɪbl kriˈeɪʃn/: phải tạo lúc tạo bảng