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I assume the values must be all be in memory beforehand and not hard storage.

That sum() SQL is all you do as a user. There is no additional magic there.

The performance numbers are indeed best when data is in memory. However in reality sum() goes over memory mapped file, lazy loading data as required.

So how would this compare to the naïve approach of using a cursor in another fast, memory-mapped database, like LMDB?