A different key for a different question
A secondary index re-keys your items so you can Query them on a new axis.
Why
The base table groups tickets by project. But 'every ticket assigned to Ada, across all projects' needs a different partition key, and you cannot change the table's keys.
A global secondary index is the answer: it is an automatically-maintained, re-keyed copy of your items. Query the base table for a project; Query the index for an assignee. Same items, two axes.
An index is a re-keyed copy of your data. Add one when an access pattern needs a key the table does not have.
These rows are real and already here. Running an operation against them downloads a DynamoDB-compatible engine, about 850 KB, and executes it in this tab.
The base table is keyed by project, so 'this project's tickets' is a Query on the table.
Run an operation to see the raw engine response.