Re: doc: explain pgstatindex fragmentation

Hi Frédéric,

On Thu, Jan 23, 2025 at 10:00:27AM +0100, Frédéric Yhuel wrote:
> On 1/22/25 12:34, Bertrand Drouvot wrote:
> > I'm not sure it's good to describe something as the inverse of "something
> > else". See my proposal below.
> >
>
> Yeah... bloat is a more familiar concept, so I wanted to link these two
> metrics

Yeah but in the (rare?) case "bloat" is not known then one would have to make
sense of it first.

> > I’m not sure we need to add the extra details in a paragraph below the fields
> > description. What about changing the fields description?
> >
> > Something concise enough like?
> >
> > avg_leaf_density: shows how full leaf pages currently are (100 if full)
>
> That should do :-)

Thanks!

> > leaf_fragmentation: shows how much physical and logical ordering of leaf pages
> > differ (zero if they don't)
> >
>
> It looks good to me.

Thanks!

> I've noticed that maximum leaf_fragmentation can have a huge impact on a
> range index-only scan, when reading all blocs from disks, even on my laptop
> machine with SSD, but I don't know if this is the right place to document
> this?

Yeah, that might be worth to mention. Maybe below the descriptions then? (keeping
the changes above in the description).

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com