user: what is the global secondary index?

github copilot: # global secondary index (gsi) explained

a gsi is an alternate way to query your dynamodb table using different keys than the primary key.

the problem without gsi

your table’s primary key is deviceid:

• ✅ you can query: “get device with id abc123”
• ❌ you cannot query: “get all devices owned by user john@example.com”

dynamodb can only query by primary key. to find a user’s devices without a gsi, you’d have to scan the entire table (slow and expensive).

the solution: gsi

a gsi creates an alternate index using userid as the key:

primary table (indexed by deviceid):
deviceid    userid              devicename
─────────────────────────────────────────
abc123      john@example.com    fluffy's
xyz789      jane@example.com    mittens'
def456      john@example.com    whiskers'

gsi: userid-index (indexed by userid):
userid              deviceid    devicename
─────────────────────────────────────────
jane@example.com    xyz789      mittens'
john@example.com    abc123      fluffy's
john@example.com    def456      whiskers'

now you can efficiently query: “get all devices for john@example.com” → returns 2 devices instantly!

your use case

when a user logs into your app:

1. get their userid from cognito
2. query the userid-index gsi
3. get all their devices in milliseconds
4. display their device list

without gsi: scan 100,000 devices to find theirs (slow, expensive)
with gsi: direct lookup of only their devices (fast, cheap)

that’s why we need it! ready to create the table and add the gsi?