Re: [HACKERS] Removing [Merge]Append nodes which contain a single subpath

On Thu, Dec 7, 2017 at 5:11 AM, David Rowley
<david(dot)rowley(at)2ndquadrant(dot)com> wrote:
>
> While rebasing this today I also noticed that we won't properly detect
> unique joins in add_paths_to_joinrel() as we're still testing for
> uniqueness against the partitioned parent rather than the only child.
> This is likely not a huge problem since we'll always get a false
> negative and never a false positive, but it is a missing optimisation.
> I've not thought of how to solve it yet, it's perhaps not worth going
> to too much trouble over.
>

It's only the jointrelids that are parent's relids, but all the
following tests for unique join use RelOptInfo::relids which are child
relids.

case JOIN_UNIQUE_INNER:
extra.inner_unique = bms_is_subset(sjinfo->min_lefthand,
outerrel->relids);
break;
case JOIN_UNIQUE_OUTER:
extra.inner_unique = innerrel_is_unique(root,
outerrel->relids,
innerrel,
JOIN_INNER,
restrictlist,
false);
break;
default:
extra.inner_unique = innerrel_is_unique(root,
outerrel->relids,
innerrel,
jointype,
restrictlist,
false);
break;

Do you have a testcase, which shows the problem?

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company