Re:Datomic Is Now Free
by twic on Thursday April 27, 2023 @04:44PM
(#35734284)
I'm moderately confident you could mechanically transform a time-oblivious schema into a history-preserving one, and then write a view on top of it which gave a slice at a particular time. Moderately.
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That is essentially what MVCC does.
That is an extremely good point.
Yes, although AFAIK those hidden MVCC columns (xmin, xmax?) aren't very usable from an application standpoint -- the obsoleted rows only hang around until the next VACUUM, right?
I realize you're not claiming those columns are useful from an application perspective. Just curious to know if I'm wrong and they are....
Oracle lets you use the MVCC data to query past states of the database, called "flashback":
https://docs.oracle.com/en/database/oracle/oracle-database/2...
You can configure how long the old data is kept:
https://docs.oracle.com/en/database/oracle/oracle-database/2...
Worked examples:
Wow, super informative. Thank you so much.
I meant MVCC is the proof that you can automate the transform of a schema into a versioned schema. How and if the DBMS exposes that is another concern.
The garbage collection / VACUUM part of an MVCC system is the harder part, saving all versions and querying a point in time is the easy one.