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* We store five "virtual" fields Xmin, Cmin, Xmax, Cmax, and Xvac in three
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* physical fields. Xmin and Xmax are always really stored, but Cmin, Cmax
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* are only interesting for the lifetime of the inserting and deleting
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* transaction respectively. If a tuple is inserted and deleted in the same
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* transaction, we store a "combo" command id that can be mapped to the real