Re: Performance Improvement by reducing WAL for Update Operation

On 4 January 2013 13:53, Amit Kapila <amit.kapila@huawei.com> wrote:
> On Friday, December 28, 2012 3:52 PM Simon Riggs wrote:
>> On 28 December 2012 08:07, Kyotaro HORIGUCHI
>> <horiguchi.kyotaro@lab.ntt.co.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.

OK, please explain in comments.

>> 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.

But also in comments for each major function.

Thanks

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services