Re: Performance impact of updating target columns with unchanged values ON CONFLICT

Thanks Justin. Do you know if Postgres treats an UPDATE that sets the
indexed columns set to the same previous values as a change? Or does it
only count it as "changed" if the values are different. This is ambiguous
to me.

*> HOT solves this problem for a restricted but useful special case where a
tuple is repeatedly updated in ways that do not change its indexed columns.*

*> With HOT, a new tuple placed on the same page and with all indexed
columns the same as its parent row version does not get new index entries.*

*> [HOT] will create a new physical heap tuple when inserting, and not a
new index tuple, if and only if the update did not affect indexed columns.*

On Thu, Nov 22, 2018 at 2:40 PM Justin Pryzby <pryzby(at)telsasoft(dot)com> wrote:

> On Thu, Nov 22, 2018 at 01:31:10PM -0800, Abi Noda wrote:
> > In other words, is Postgres smart enough to not actually write to disk
> any
> > columns that haven’t changed value or update indexes based on those
> columns?
>
> You're asking about what's referred to as Heap only tuples:
>
> https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/access/heap/README.HOT;hb=HEAD
> https://wiki.postgresql.org/wiki/Index-only_scans#Interaction_with_HOT
>
> Note, if you're doing alot of updates, you should consider setting a lower
> the
> table fillfactor, since HOT is only possible if the new tuple (row
> version) is
> on the same page as the old tuple.
>
> |With HOT, a new tuple placed on the same page and with all indexed
> columns the
> |same as its parent row version does not get new index entries."
>
> And check pg_stat_user_tables to verify that's working as intended.
>
> Justin
>