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

On Thu, Feb 23, 2017 at 03:26:09PM -0300, Alvaro Herrera wrote:
> Bruce Momjian wrote:
> > On Thu, Feb 23, 2017 at 03:03:39PM -0300, Alvaro Herrera wrote:
> > > Bruce Momjian wrote:
> > > 
> > > > As I remember, WARM only allows
> > > > a single index-column change in the chain.  Why are you seeing such a
> > > > large performance improvement?  I would have thought it would be that
> > > > high if we allowed an unlimited number of index changes in the chain.
> > > 
> > > The second update in a chain creates another non-warm-updated tuple, so
> > > the third update can be a warm update again, and so on.
> > 
> > Right, before this patch they would be two independent HOT chains.
> 
> No, they would be a regular update chain, not HOT updates.

Well, let's walk through this.  Let's suppose you have three updates
that stay on the same page and don't update any indexed columns --- that
would produce a HOT chain of four tuples.  If you then do an update that
changes an indexed column, prior to this patch, you get a normal update,
and more HOT updates can be added to this.  With WARM, we can join those
chains and potentially trim the first HOT chain as those tuples become
invisible.

Am I missing something?

--  Bruce Momjian          http://momjian.us EnterpriseDB                             http://enterprisedb.com

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