Foundations · Lesson 2 of 20

The key is the address

GetItem needs the whole primary key: partition and sort.

Next · Reads and costOne query, a whole collection→

Why

This table has a composite key: a partition key (pk, the room) and a sort key (sk, the message). The partition key decides which partition an item lives on; the sort key orders items within that partition.

GetItem fetches exactly one item, and it needs the whole key. Knowing only the room is not enough to GetItem a message. That is a job for Query, next.

Know the full key, pay for one item. GetItem is the cheapest read DynamoDB offers.

Chatdynoxide · wasm · DynamoDB API 2012-08-10 · contract 1

Try an example

The full key: the room (partition) and the message id (sort). One item, the cheapest read there is.

Indexoptionalbase table, or a secondary indexKeyConditionExpressionFilterExpressionoptionalExpressionAttributeValues

pk(pk)

sk(sk)

topic

from

text

Run an operation to see the raw engine response.

table Chatkeys pk / skitems 22