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0 pointsysleepy2y ago0 comments

That is essentially what MVCC does.

0 comments

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.

ysleepyOP2y ago

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.

twic2y ago

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

JohnBooty2y ago

Wow, super informative. Thank you so much.

twic2y ago

That is an extremely good point.

j / k navigate · click thread line to collapse

0 comments

JohnBooty2y ago

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.

ysleepyOP2y ago

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.

twic2y ago

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

JohnBooty2y ago

Wow, super informative. Thank you so much.

twic2y ago

That is an extremely good point.

j / k navigate · click thread line to collapse