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In https://github.com/neondatabase/neon/issues/9441, the tenant has a lot of aux keys spread in multiple aux files. The perf tool shows that a significant amount of time is spent on remove_overlapping_keys. For sparse keyspaces, we don't need to report missing key errors anyways, and it's very likely that we will need to read all layers intersecting with the key range. Therefore, this patch adds a new fast path for sparse keyspace reads that we do not track `unmapped_keyspace` in a fine-grained way. We only modify it when we find an image layer. In debug mode, it was ~5min to read the aux files for a dump of the tenant, and now it's only 8s, that's a 60x speedup. ## Summary of changes * Do not add sparse keys into `keys_done` so that remove_overlapping does nothing. * Allow `ValueReconstructSituation::Complete` to be updated again in `ValuesReconstructState::update_key` for sparse keyspaces. --------- Signed-off-by: Alex Chi Z <chi@neon.tech>