Re: questions about PG update performance

On Mon, Oct 26, 2015 at 12:07 PM, Ashutosh Bapat <
ashutosh(dot)bapat(at)enterprisedb(dot)com> wrote:

>
>
> On Mon, Oct 26, 2015 at 10:19 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
>>
>>
>> I think Oracle just copies the changed part of old row to rollback
>> segment.
>> Also in Redo logs, it just writes the changed column value (both old and
>> new). So for the case we are discussing in this thread (one changed
>> column out of 200 columns), Oracle will just write the old value of that
>> column in Redo and then in rollback segment, and write the new value
>> in Redo and then do the in-place update in heap row.
>>
>>
> In that case, readers would pay the penalty for constructing the row.
>

Readers that have snapshot older than update-transaction needs to
pay such cost, otherwise all newer transactions can directly read from
page. Also not all old-transaction readers have to pay any such cost.

Not only that, such a design has an advantage that the bloat due to
older data won't be there.

With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com