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Re: Performance Improvement by reducing WAL for Update Operation

Amit Kapila <amit.kapila@huawei.com>

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From: Amit kapila <amit.kapila@huawei.com>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>, "hlinnakangas@vmware.com" <hlinnakangas@vmware.com>, "noah@leadboat.com" <noah@leadboat.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2013-01-11T18:11:11Z
Lists: pgsql-hackers 
On Friday, January 11, 2013 11:09 PM Simon Riggs wrote:
On 11 January 2013 17:08, Amit kapila <amit.kapila@huawei.com> wrote:

>>> Just reviewing the patch now, making more sense with comments added.
>
>>>In heap_delta_encode() do we store which columns have changed?
>
>> Not the attribute bumberwise, but offsetwise it is stored.

> (Does that mean "numberwise"??)
   Yes.

> Can we identify which columns have changed? i.e. 1st, 3rd and 12th columns?
  As per current algorithm, we can't as it is based on offsets.
  What I mean to say is that the basic idea to reconstruct tuple during recovery 
  is copy data from old tuple offset-wise (offsets stored in encoded tuple) and use new data (modified column data)
  from encoded tuple directly. So we don't need exact column numbers.

With Regards,
Amit Kapila.