What it is: A secondary index with a different partition key (and optional sort key) than the base table.

What it’s for:

• Query the table using alternate keys (different access patterns).

Key ideas:

• “Global” means it spans all partitions (not limited to one partition key like LSI).
• GSIs can be added after table creation.
• GSIs are eventually consistent by nature (common expectation; depends on usage, but exam treats them as flexible).

Exam cues:

• “need to query by email instead of userId” → add GSI on email.

Hard words:

• *index* /ˈɪndeks/: chỉ mục
• *alternate* /ˈɔːltərnət/: thay thế/khác
• *eventually consistent* /ɪˈventʃuəli kənˈsɪstənt/: nhất quán sau (có độ trễ đồng bộ)