[bvanderveen]

We implemented it in Postgres with 'created_at' and 'deleted_at' columns on everything and filtering to make sure that the object 'exists' at the time the query is concerned with. Changes in relationships between objects are modeled as join tables with a boolean indicating whether the relationship is made or broken and at what time.

Our data model is not large and we had a very complete test suite already, so it was easy to produce another implementation backed by postgres, RAM, etc.

[wwweston]

Yeah, it seems you could be able to substitute thoughtful schema design avoiding updates/deletes for time-travel as a feature.

I wonder if anyone has made a collection of reference examples implemented this way (and in general think that a substantial compendium good examples of DB schema and thinking behind them could be worthwhile).

[pjot]

It’s called a slowly changing dimension. In this example, it’s a type-2.

https://en.m.wikipedia.org/wiki/Slowly_changing_dimension

[twic]

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.

[ysleepy]

That is essentially what MVCC does.

[JohnBooty]

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 useful.

Because as I understand it, the selling point of Datomic is their audit trail functionality and that is admittedly a bit onerous to implement in a RBDMS. Even though I feel like every project needs/requires that eventually.

[ysleepy]

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.

[twic]

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:

http://www.dba-oracle.com/t_rman_149_flasbback_query.htm

[JohnBooty]

Wow, super informative. Thank you so much.

[twic]

That is an extremely good point.

[mjul]

Snodgrass wrote a whole book on that topic: Developing Time-Oriented Database Applications in SQL (1999).

It is available as PDF on his publications page:

https://www2.cs.arizona.edu/~rts/publications.html

[rehevkor5]

Maybe search around on bitemporal database table modeling.

[jskulski]

Ive built a couple systems that would have been datomic’s bread and butter.

Each time the company was more comfortable with mainstream dbs, so we ended going with something like you’re talking about, built on top of a db. A couple of the projects were because a mainstream dbs wouldn’t scale.

The systems definitely worked, but it was also a lot of implementation complexity on an other wise simple business prop: “store this data as facts”