Re: UNDO and in-place update

On Wed, Nov 23, 2016 at 9:32 AM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> On Tue, Nov 22, 2016 at 7:31 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> - Reading a page that has been recently modified gets significantly
>>> more expensive; it is necessary to read the associated UNDO entries
>>> and do a bunch of calculation that is significantly more complex than
>>> what is required today.
>
> Someone told me that there is something called an interested
> transaction list stored in the page header, and from that I infer that
> isn't *really* true. I think that unless you're interested in an
> affected row, rather than just some row that happens to be on the same
> page, you don't really have to worry about it.
>

Yeah, so basically if there is an effect of any transaction which is
not visible to the snapshot of transaction reading the page, you need
to do something to read the old row/rows present on that page.

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