4 ms·

Happy to! It's not impossible for one instance to be a subset of the other. They can also be mutually exclusive periods of time. Let's do an example to explore

by gen220 7y ago

Happy to! It's not impossible for one instance to be a subset of the other. They can also be mutually exclusive periods of time. Let's do an example to explore Let's say that the present concept of John's address is "123 Apple St"; this was inserted into the database at 1/1/2020. We don't have credible evidence for what John's address was prior to 1/1/2020. Therefore, the row looks like this: > (address:"123 Apple St", valid_period:[1/1/2020,+inf), transaction_period:[1/1/2020,+inf)) This is a case where transaction period and valid period are equal. If John sent us this information by mail, and he signed it 12/25/2019, we have credible evidence that this was John's address, at least effective 12/25/2019. Therefore, his row would look like this: (address:"123 Apple St", valid_period:[12/25/2019,+inf), transaction_period:[1/1/2020,+inf)). Now, the transaction period is still 1/1/2020, because this information found it's way into the database on 1/1/2020. Now, let's continue with the second scenario. Let's say we get a second letter from John, processed on 6/1/2020, saying that he moved to "456 Orange St" on 5/1/2020. There are now two rows in the database, as below: > (address:"123 Apple St", vp:[12/25/2019,5/1/2020), tp:[1/1/2020,+inf)) > (address:"456 Orange St", vp:[5/1/2020,+inf), tp:[6/1/2020,+inf)) Then, we receive a FINAL letter from John, processed on 7/1/2020, revealing that his previous letter contained a typo! it was "789 Orange St", not "456"! Darn. Our table now contains three rows: > (address:"123 Apple St", vp:[12/25/2019,5/1/2020), tp:[1/1/2020,+inf)) > (address:"456 Orange St", vp:[5/1/2020,+inf), tp:[6/1/2020,7/1/2020)) > (address:"456 Orange St", vp:[5/1/2020,+inf), tp:[7/1/2020,+inf)) Let me know if you have any questions about this example!

andrewflnr 7y ago
Ok, so the validity period tries to describe the real world, while the tp is more about the state of the database's knowledge, or rather belief. I think I understand how each one starts when it does, but not when you update the endings. - Does the validity period of the erroneous entry ever get closed? - Or the transaction period of the one that got superseded? - Do transaction periods close for reasons other than finding out something was wrong, or can I think of them as the period that the rest of the row (including validity) is/was believed? Thanks for the detailed explanation. This is really interesting.
- gen220 7y ago
  No problem! It’s fun! And a powerful conceptual tool. In my experience, it can be used to solve many thorny problems, and I never learned about it in school. I had a typo in the last row, it should have been > (“789 Orange St”, vp:(5/1/2020, +inf), tp:(7/1/2020, +inf)) my apologies! - the valid period of the middle record is never closed, because it would be a misrepresentation of how the database’s perception of the address at that point in ”transaction time”. - indeed, the superseded transaction period is closed. - formally, transaction periods close (and new rows are created) whenever a column’s value changes. Think of it like an “updated at” time stamp, except the meaning is more like “canonical during”. The second half of your question is totally correct, the TP is the period during which the values representing the specified slice of valid time are/were believed. As the third bullet point implies, one characteristic of this schema is that you have “non-destructive” updates: state is never lost, it’s just put into transaction history. This makes it possible to “roll back” to an earlier, known-good state: simply specify a point in transaction time. There’s a whole additional rabbit hole to dive down: how to make this stuff fast and intuitive. People have mostly solved the fast part, but we’re still working on the intuition.
  - madno 7y ago
    For total ordering are you using an instant (with say seconds granularity) or an additional monotone id to distinguish between changes for the same day?
  - andrewflnr 7y ago
    I meant to ask about the transaction period of the first row (my bad, "superseded" is ambiguous), but I think I get that one now: the transaction period is still open because we still believe (and probably will forever) that that's the correct validity period for that address. Is that right? And the validity period for the middle one never closes because there's never a time in that row's transaction period where we know the end of the validity period. The intuition I'm building is that the validity period is metadata and the transaction period is metametadata. I think the similarity between them is a little deceptive, since the validity period is in some sense just another piece of data sort of controlled by (or interpreted in light of) the transaction period, which is itself a way to simulate versioning the whole database; I think of it like the internal representation for a database wrapper that lets you time travel in a database with a history-oblivious schema. In that vision, validity periods are actually part of the history-oblivious schema. You could totally have "transaction periods" for stuff with no concept of validity period, e.g. something timeless where our understanding evolves over time. If you're trying to teach people, I'd suggest introducing the two concepts independently, then showing the interplay with an example like the one you gave here. I don't know if weird nested contexts can ever be really "intuitive". :D

by gen220 7y ago

andrewflnr 7y ago

Ok, so the validity period tries to describe the real world, while the tp is more about the state of the database's knowledge, or rather belief. I think I understand how each one starts when it does, but not when you update the endings. - Does the validity period of the erroneous entry ever get closed? - Or the transaction period of the one that got superseded? - Do transaction periods close for reasons other than finding out something was wrong, or can I think of them as the period that the rest of the row (including validity) is/was believed? Thanks for the detailed explanation. This is really interesting.

gen220 7y ago
No problem! It’s fun! And a powerful conceptual tool. In my experience, it can be used to solve many thorny problems, and I never learned about it in school. I had a typo in the last row, it should have been > (“789 Orange St”, vp:(5/1/2020, +inf), tp:(7/1/2020, +inf)) my apologies! - the valid period of the middle record is never closed, because it would be a misrepresentation of how the database’s perception of the address at that point in ”transaction time”. - indeed, the superseded transaction period is closed. - formally, transaction periods close (and new rows are created) whenever a column’s value changes. Think of it like an “updated at” time stamp, except the meaning is more like “canonical during”. The second half of your question is totally correct, the TP is the period during which the values representing the specified slice of valid time are/were believed. As the third bullet point implies, one characteristic of this schema is that you have “non-destructive” updates: state is never lost, it’s just put into transaction history. This makes it possible to “roll back” to an earlier, known-good state: simply specify a point in transaction time. There’s a whole additional rabbit hole to dive down: how to make this stuff fast and intuitive. People have mostly solved the fast part, but we’re still working on the intuition.
- madno 7y ago
  For total ordering are you using an instant (with say seconds granularity) or an additional monotone id to distinguish between changes for the same day?
- andrewflnr 7y ago
  I meant to ask about the transaction period of the first row (my bad, "superseded" is ambiguous), but I think I get that one now: the transaction period is still open because we still believe (and probably will forever) that that's the correct validity period for that address. Is that right? And the validity period for the middle one never closes because there's never a time in that row's transaction period where we know the end of the validity period. The intuition I'm building is that the validity period is metadata and the transaction period is metametadata. I think the similarity between them is a little deceptive, since the validity period is in some sense just another piece of data sort of controlled by (or interpreted in light of) the transaction period, which is itself a way to simulate versioning the whole database; I think of it like the internal representation for a database wrapper that lets you time travel in a database with a history-oblivious schema. In that vision, validity periods are actually part of the history-oblivious schema. You could totally have "transaction periods" for stuff with no concept of validity period, e.g. something timeless where our understanding evolves over time. If you're trying to teach people, I'd suggest introducing the two concepts independently, then showing the interplay with an example like the one you gave here. I don't know if weird nested contexts can ever be really "intuitive". :D