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The undo_space parameter only changes behavior during the EnableVersioning process. It makes no difference after the table has been versioned. All transactions work as they normally do with regard to undo. During EnableVersioning, we need to update the entire base table. In order to accommodate smaller undo tablespaces, we allow the user to specify the maximum amount of undo_space that should be used. Based on that, we update a subset of the table making sure to not exceed the specified amount. Each of these updates is performed in its own transaction.