Re: questions about PG update performance

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

On Mon, Oct 26, 2015 at 10:19 AM, Amit Kapila <amit.kapila16@gmail.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.