From: | Justin Pryzby <pryzby(at)telsasoft(dot)com> |
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To: | Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at> |
Cc: | David Rowley <dgrowleyml(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Darafei Komяpa Praliaskouski <me(at)komzpa(dot)net>, Andres Freund <andres(at)anarazel(dot)de>, pgsql-hackers(at)lists(dot)postgresql(dot)org, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Michael Banck <mbanck(at)gmx(dot)net> |
Subject: | Re: Berserk Autovacuum (let's save next Mandrill) |
Date: | 2020-03-16 04:34:25 |
Message-ID: | 20200316043424.GF26184@telsasoft.com |
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Lists: | pgsql-hackers |
On Fri, Mar 13, 2020 at 10:48:27PM +0100, Laurenz Albe wrote:
> On Fri, 2020-03-13 at 13:44 -0500, Justin Pryzby wrote:
> > Possible it would be better to run VACUUM *without* freeze_min_age=0 ? (I get
> > confused and have to spend 20min re-reading the vacuum GUC docs every time I
> > deal with this stuff, so maybe I'm off).
> >
> > As I understand, the initial motivation of this patch was to avoid disruptive
> > anti-wraparound vacuums on insert-only table. But if vacuum were triggered at
> > all, it would freeze the oldest tuples, which is all that's needed; especially
> > since fd31cd2651 "Don't vacuum all-frozen pages.", those pages would never need
> > to be vacuumed again. Recently written tuples wouldn't be frozen, which is ok,
> > they're handled next time.
>
> Freezing tuples too early is wasteful if the tuples get updated or deleted
> soon after, but based on the assumption that an autovacuum triggered by insert
> is dealing with an insert-mostly table, it is not that wasteful.
You're right that it's not *that* wasteful. If it's a table that gets 90%
inserts/10% updates, then only 10% of its tuples will be frozen. In the worst
case, it's the same tuples every time, and that's somewhat wasteful. In the
best case, those tuples are clustered on a small number of pages.
--
Justin
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