Absence as a filter
An item appears in an index only if it has the index's key, so leaving the key off filters it out.
Why
An item is copied into a secondary index only if it has every attribute in that index's key. In this table only open tickets are given a gsi1pk, so only open tickets exist in the by-assignee index.
Scan the index, then the table. The index is a ready-made list of open work, with no filter expression and nothing wasted. Close a ticket by removing its gsi1pk and it drops out of the index on its own.
A sparse index uses absence as a global filter: only the items you care about ever appear in it.
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.
Scan the by-assignee index. Only OPEN tickets carry gsi1pk, so closed tickets are simply not here, even though they exist in the table.
Run an operation to see the raw engine response.