On Wed, Mar 16, 2016 at 2:25 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
> Robert Haas <robertmh...@gmail.com> writes:
>> On Wed, Mar 16, 2016 at 2:02 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
>>> Looks pretty close.  One point is that if we do end up using a Result
>>> node, then the parent GatherPath does not get charged for the Result
>>> node's cpu_per_tuple overhead.  I'm not sure that that's worth changing
>>> though.  It's probably better to bet that the subpath is projectable and
>>> so no cost will ensue, than to bet the other way.
>> I'm almost sure this way is the better bet.
> Actually, we do know what will happen ... so maybe
>         /*
>          * We always use create_projection_path here, even if the subpath is
>          * projection-capable, so as to avoid modifying the subpath in place.
>          * It seems unlikely at present that there could be any other
>          * references to the subpath anyway, but better safe than sorry.
>          */
> +       if (!is_projection_capable_path(gpath->subpath))
> +           gpath->path.total_cost += cpu_tuple_cost * gpath->subpath->rows;
>         gpath->subpath = (Path *)
>             create_projection_path(root,
>                                    gpath->subpath->parent,
>                                    gpath->subpath,
>                                    target);
> The comment could use adjustment if you adopt that, to reference the fact
> that we know create_projection_plan will get rid of the Result if not
> needed.

OK, I've committed something along those lines.  Thanks for the
advice, and feel free to whack it around if you have an idea for
improving it further - though IMHO this is good enough for 9.6.

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:

Reply via email to