Re: [HACKERS] Patch: Write Amplification Reduction Method (WARM)

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@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com

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