Re: Fix BUG #17335: Duplicate result rows in Gather node

On Tue, Feb 8, 2022 at 1:11 PM David Rowley <dgrowleyml@gmail.com> wrote:

> Thanks for having a look at this.
>
> On Fri, 4 Feb 2022 at 13:48, Robert Haas <robertmhaas@gmail.com> wrote:
> > I think the actual rule is: every path under a Gather or GatherMerge
> > must be parallel-safe.
>
> I've adjusted the patch so that it counts parallel_aware and
> parallel_safe Paths independently and verifies everything below a
> Gather[Merge] is parallel_safe.
>
> The diff stat currently looks like:
>
> src/backend/optimizer/plan/createplan.c | 230
> 1 file changed, 230 insertions(+)
>
> I still feel this is quite a bit of code for what we're getting here.
> I'd be more for it if the path traversal function existed for some
> other reason and I was just adding the callback functions and Asserts.
>
> I'm keen to hear what others think about that.
>
> David
>
Hi,

+           break;
+           case T_MergeAppend:

The case for T_MergeAppend should be left indented.

+       case T_Result:
+           if (IsA(path, ProjectionPath))

Since the remaining sub-cases don't have subpath, they are covered by the
final `else` block - MinMaxAggPath and GroupResultPath don't need to be
checked.

For contains_a_parallel_aware_path(), it seems path_type_counter() can
return bool indicating whether the walker should return early (when
parallel aware count reaches 1).

Cheers