How are queries identified with keepPreviousData

[mwisnicki]

As per https://tanstack.com/query/v4/docs/guides/paginated-queries when none sets keepPreviousData:

The data from the last successful fetch available while new data is being requested, even though the query key has changed.

If the query key is ignored how can the QueryClient identify old data in the cache?

[TkDodo]

per observer. Every useQuery call creates an observer that knows it's previous state.