Re: On markers of changed data

> Le 6 oct. 2017 à 23:44, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> a écrit :
>
> Michael Paquier wrote:
>
>> The only sane method for Postgres is really to scan the
>> page header LSNs, and of course you already know that.
>
> I hope the idea is not to have to scan every single page in the
> database, because that would be too slow. It should be possible to
> build this so that a single summary LSN is kept for a largish group of
> pages, allowing large portions of the database to be skipped from even
> being read if they are known not to contain any page newer than the
> previous backup.

That’s actually what pg_rman is doing for what it calls incremental backups (perhaps that would be differential backup in PG terminology?), and the performance is bad as you can imagine. We could have a dedicated LSN map to do such things with 4 bytes per page. I am still not convinced that this much facility and the potential bug risks are worth it though, Postgres already knows about differential backups if you shape it as a delta of WAL segments. I think that, in order to find a LSN map more convincing, we should find first other use cases where it could become useful. Some use cases may pop up with VACUUM, but I have not studied the question hard enough...
--
Michael