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Re: Performance Improvement by reducing WAL for Update Operation

From: Amit Kapila <amit(dot)kapila(at)huawei(dot)com>
To: "'Simon Riggs'" <simon(at)2ndQuadrant(dot)com>, "'Kyotaro HORIGUCHI'"<horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: <hlinnakangas(at)vmware(dot)com>, <noah(at)leadboat(dot)com>, <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Performance Improvement by reducing WAL for Update Operation
Date: 2013-01-04 13:53:58
Message-ID: 00a001cdea82$f29743a0$d7c5cae0$ (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Friday, December 28, 2012 3:52 PM Simon Riggs wrote:
> On 28 December 2012 08:07, Kyotaro HORIGUCHI
> <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> > Hello, I saw this patch and confirmed that
> >
> >  - Coding style looks good.
> >  - Appliable onto HEAD.
> >  - Some mis-codings are fixed.
> I've had a quick review of the patch to see how close we're getting.
> The perf tests look to me like we're getting what we wanted from this
> and I'm happy with the recovery performance trade-offs. Well done to
> both author and testers.
> * The compression algorithm depends completely upon new row length
> savings. If the new row is short, it would seem easier to just skip
> the checks and include it anyway. We can say if old and new vary in
> length by > 50% of each other, just include new as-is, since the rows
> very clearly differ in a big way. 

> Also, if tuple is same length as 
> before, can we compare the whole tuple at once to save doing
> per-column checks?

If we have to do whole tuple comparison then storing of changed parts might
need to be
be done in a byte-by-byte way rather then at column offset boundaries.
This might not be possible with current algorithm as it stores in WAL
information column-by-column and decrypts also in similar way.

> The internal docs are completely absent. We need at least a whole page of
descriptive > comment, discussing trade-offs and design decisions.

Currently I have planned to put it transam/README, as most of WAL
description is present there.

With Regards,
Amit Kapila.

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