I am currently exploring Change Data Capture as an option to store temporal databases. It is great because it stores only the deltas and seems like it may solve my problem. When I enabled CDC, a bunch of tables appeared under System Tables
.
When querying cdc.dbo_MyTable
, I am able to see all the changes that took place on the table. Now, how would I construct a historical view? For instance, if I wanted to see the state of my table as of a particular date, how would I go about doing that? Is that even possible?
It looks I need to take the log and start applying it over my original table but I was wondering if there is a built-in way of doing this. Any suggestions?
Some of the use cases I am looking at:
it's possible, but not with a built-in way i'm a afraid. You would have to reconstruct the timeline by hand.
Given that the change-tracking tables offer the tran_end_time, which is the time that the value of the property should be perceived as persisted, you would have to make a query that fetches all the distinct periods of table states, join on the tracked property changes and then pivot (to have a presentation in the same form as the table). Don't forget to union with the table state itself to obtain the values that have not been changed/tracked for completeness.
The final result, simplified, should look like
RN PK PropA PropB FromDate ToDate
1 1 'Ver1' 'Ver1' 2012-01-01 09:00 2012-01-02 08:00
2 1 'Ver1' 'Ver2' 2012-01-02 08:00 2012-01-03 07:00
3 1 'Ver2' 'Ver2' 2012-01-03 07:00 *getdate()*
4 2 'Ver1' 'Ver1' 2012-01-01 05:00 2012-01-02 06:00
5 2 'Ver1' 'Ver2' 2012-01-02 06:00 2012-01-03 01:00
6 2 'Ver2' 'Ver2' 2012-01-03 01:00 *getdate()*
note that the getdate() is valid if the row wasn't deleted in which case it should be substituted with the deletion date
EDIT, for the 2 use cases. The first point is easily addressed it's a matter of constructing the temporal object graph and then filtering:
declare @pointInTime datetime = '20120102 10:00';
select * from Reconstructed_TG where FromDate <= @pointInTime and @pointInTime < ToDate
the second point, can be generated easily with the EXCEPT clause, as you point out. given the above query:
declare @pointInTimeA datetime = '20120102 10:00';
declare @pointInTimeB datetime = '20120103 01:00';
select * from Reconstructed_TG where FromDate <= @pointInTimeA and @pointInTimeA < ToDate
EXCEPT
select * from Reconstructed_TG where FromDate <= @pointInTimeB and @pointInTimeB < ToDate
yet the except
clause only presents the rows that have at least one different column value; i don't know if that information is really meaningful to the human eye. Depending on your needs a query that works directly on the cdc
data may be more appropriate.
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