====== Global Secondary Index (GSI) ======

**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)