Logged Time and Stated Time
This chapter takes a much closer look at two concepts originally introduced in Chapter 4, viz., “valid time” and “transaction time.” Valid time has to do with when we believe (explicitly or implicitly) that some proposition is, was, or will be true. Transaction time has to do with when the database said (explicitly or implicitly) that we believe (again, explicitly or implicitly) that some proposition is, was, or will be true. Valid times are maintained—at least in a relational system—just like any other kind of data; in particular, they’re updatable just like any other kind of data. By contrast, transaction times are maintained by the system; in particular, therefore, they’re read-only (at least from the ...
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