Re: Visibility map and freezing

Gregory Stark <stark(at)enterprisedb(dot)com> wrote:

> >> I don't think we can perfectly capture the meaning of these GUCs in the
> >> name. I think our goal should be to avoid confusion between them.
>
> I was thinking it would be clearer if the options which control *when*
> autovacuum fires off a worker consistently had some action word in them like
> "trigger" or "start" or "launch".

I think we need more explanations about those variables,
not only "how to work" but also "how to tune" them.
I feel they are un-tunable parameters.

Our documentation says:
| Larger values of these settings
| preserve transactional information longer, while smaller values increase
| the number of transactions that can elapse before the table must be
| vacuumed again.
i.e, we are explaining the variables only as "Larger is better",
but is it really true?

I think we should have answers about the following questions:

- What relation are there between autovacuum_freeze_max_age,
vacuum_freeze_min_age and vacuum_freeze_table_age? If we increase
one of them, should we also increase the others?

- Is it ok to increase the variables to maximum values?
Are there any trade-off?

- Are there some conditions where whole-table-scanning vacuum is more
effective than vacuums using visibility map? If so, we should switch
to full-scan *automatically*, without relying on user configurations.

Regards,
---
ITAGAKI Takahiro
NTT Open Source Software Center