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In-Reply-To: <9485d892-fd04-4e3a-ac24-7dd767cb7333@postgrespro.ru>
From: Alexander Korotkov <aekorotkov@gmail.com>
Date: Mon, 28 Oct 2024 21:07:02 +0200
Message-ID: <CAPpHfduwY8-fp34CuO9O57ouCs1K=Gn1rTnuG4AaWYhEo6nXyw@mail.gmail.com>
Subject: Re: Vacuum statistics
To: Alena Rybakina <a.rybakina@postgrespro.ru>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, 
	Andrei Zubkov <zubkov@moonset.ru>, jian he <jian.universality@gmail.com>, 
	Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>, Alena Rybakina <lena.ribackina@yandex.ru>, 
	pgsql-hackers <pgsql-hackers@postgresql.org>, a.lepikhov@postgrespro.ru
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On Sun, Sep 29, 2024 at 12:22=E2=80=AFAM Alena Rybakina
<a.rybakina@postgrespro.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 vacuu=
m in relation to a table or the database as a whole.
>
> For example, there may be a situation where vacuum has started to run mor=
e often and spends a lot of resources on processing a certain index, but th=
e size of the index does not change significantly. Moreover, the table in w=
hich 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 compar=
ed to other objects, vacuum processed more blocks and spent more time on th=
is index.
>
> Perhaps the vacuum parameters for the index should be set more aggressive=
ly 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 remove=
d, 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