From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
Cc: | Pavan Deolasee <pavan(dot)deolasee(at)gmail(dot)com>, Jaime Casanova <jaime(dot)casanova(at)2ndquadrant(dot)com>, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Patch: Write Amplification Reduction Method (WARM) |
Date: | 2017-02-23 19:12:28 |
Message-ID: | 20170223191228.GN20486@momjian.us |
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Lists: | pgsql-hackers |
On Thu, Feb 23, 2017 at 03:58:59PM -0300, Alvaro Herrera wrote:
> Bruce Momjian wrote:
> > On Thu, Feb 23, 2017 at 03:45:24PM -0300, Alvaro Herrera wrote:
>
> > > > and potentially trim the first HOT chain as those tuples become
> > > > invisible.
> > >
> > > That can already happen even without WARM, no?
> >
> > Uh, the point is that with WARM those four early tuples can be removed
> > via a prune, rather than requiring a VACUUM. Without WARM, the fourth
> > tuple can't be removed until the index is cleared by VACUUM.
>
> I *think* that the WARM-updated one cannot be pruned either, because
> it's pointed to by at least one index (otherwise it'd have been a HOT
> update). The ones prior to that can be removed either way.
Well, if you can't prune across index-column changes, how is a WARM
update different than just two HOT chains with no WARM linkage?
> I think the part you want (be able to prune the WARM updated tuple) is
> part of what Pavan calls "turning the WARM chain into a HOT chain", so
> not part of the initial patch. Pavan can explain this part better, and
> also set me straight in case I'm wrong in the above :-)
VACUUM can already remove entire HOT chains that have expired. What
his VACUUM patch does, I think, is to remove the index entries that no
longer point to values in the HOT/WARM chain, turning the chain into
fully HOT, so another WARM addition to the chain can happen.
--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ As you are, so once was I. As I am, so you will be. +
+ Ancient Roman grave inscription +
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