Re: Autovacuum / full vacuum

Hi, Michael,

Michael Riess wrote:

> But actually I never understood why the database system slows down at
> all when there is much unused space in the files. Are the unused pages
> cached by the system, or is there another reason for the impact on the
> performance?

No, they are not cached as such, but PostgreSQL caches whole pages, and
you don't have only empty pages, but also lots of partially empty pages,
so the signal/noise ratio is worse (means PostgreSQL has to fetch more
pages to get the same data).

Sequential scans etc. are also slower.

And some file systems get slower when files get bigger or there are more
files, but this effect should not really be noticeable here.

HTH,
Markus

--
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf. | Software Development GIS

Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org