The last cache can be stored as a map of database/table/cache (there can be multiple named caches per table).

Each cache has a set of key columns (this is described well in this comment: #25109 (review)), and itself is a nested map, with one level for each key column in the hash. The nesting is done to create a hierarchy from the key columns if the cache.

Writes to the last cache will be sent after they have been flushed to the WAL, i.e., here.

The last cache needs to have an age out, or time-to-live (TTL) so that cache entries beyond a given age (default to 4 hours) are evicted, and the cache isn't bloated with old data.

The last cache implementation needs to also implement Datafusion's TableProvider trait, but that work may make more sense along with #25095.