How would you do a rollback if an error occurred mid-query? How would you keep your table from being a big pile of junk if a power failure happened during the query?
As most non-MVCC database do: by writing WAL records.
In theory, it seems to me that we could implement an overwriting storage manager for PostgreSQL (i.e. we'd have both MVCC and non-MVCC, and allow users to choose at runtime). It would require a *lot* of work, but I can't see any fundamental reason why it wouldn't be possible.
-Neil
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