Re: [HACKERS] Partition-wise aggregation/grouping

On Mon, Feb 26, 2018 at 6:38 AM, Jeevan Chalke
<jeevan(dot)chalke(at)enterprisedb(dot)com> wrote:
> One idea I thought about is to memcpy the struct once we have set all
> required fields for grouped_rel so that we don't have to do similar stuff
> for partially_grouped_rel.

I think that would be a poor idea. We want to copy a few specific
fields, not everything, and copying those fields is cheap, because
they are just simple assignment statements. I think memcpy()'ing the
whole structure would be using a sledgehammer to solve a problem for
which a scalpel is more suited.

> Paths in pathlist are added by add_path(). Though we have paths is pathlist
> is sorted with the cheapest total path, we generally use
> RelOptInfo->cheapest_total_path instead of using first entry, unlike partial
> paths. But here you use the first entry like partial paths case. Will it
> better to use cheapest total path from partially_grouped_rel? This will
> require calling set_cheapest on partially_grouped_rel before we call this
> function.

Hmm, I guess that seems like a reasonable approach, although I am not
sure it matters much either way.

> Attached top-up patch doing this along with few indentation fixes.

I don't see much point to the change in generate_gather_paths -- that
line is only 77 characters long.

Committed after incorporating your other fixes and updating the
optimizer README.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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