Re: Vacuum statistics

On Sun, Sep 29, 2024 at 12:22 AM Alena Rybakina
<a(dot)rybakina(at)postgrespro(dot)ru> wrote:
> Hi! Thank you for your interesting for this patch!
>
> I took a very brief look at this and was wondering if it was worth
> having a way to make the per-table vacuum statistics opt-in (like a
> table storage parameter) in order to decrease the shared memory
> footprint of storing the stats.
>
> I'm not sure how users can select tables that enable vacuum statistics
> as I think they basically want to have statistics for all tables, but
> I see your point. Since the size of PgStat_TableCounts approximately
> tripled by this patch (112 bytes to 320 bytes), it might be worth
> considering ways to reduce the number of entries or reducing the size
> of vacuum statistics.
>
> The main purpose of these statistics is to see abnormal behavior of vacuum in relation to a table or the database as a whole.
>
> For example, there may be a situation where vacuum has started to run more often and spends a lot of resources on processing a certain index, but the size of the index does not change significantly. Moreover, the table in which this index is located can be much smaller in size. This may be because the index is bloated and needs to be reindexed.
>
> This is exactly what vacuum statistics can show - we will see that compared to other objects, vacuum processed more blocks and spent more time on this index.
>
> Perhaps the vacuum parameters for the index should be set more aggressively to avoid this in the future.
>
> I suppose that if we turn off statistics collection for a certain object, we can miss it. In addition, the user may not enable the parameter for the object in time, because he will forget about it.

I agree with this point. Additionally, in order to benefit from
gatherting vacuum statistics only for some relations in terms of
space, we need to handle variable-size stat entries. That would
greatly increase the complexity.

> As for the second option, now I cannot say which statistics can be removed, to be honest. So far, they all seem necessary.

Yes, but as Masahiko-san pointed out, PgStat_TableCounts is almost
tripled in space. That a huge change from having no statistics on
vacuum to have it in much more detail than everything else we
currently have. I think the feasible way might be to introduce some
most demanded statistics first then see how it goes.

------
Regards,
Alexander Korotkov
Supabase