Re: pg_plan_advice

Hi Robert,

On Wed, Jan 28, 2026 at 9:14 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> Thanks to all who have reviewed so far, and please keep it coming. I
> am especially in need of more code review at this point.
>

Thanks for the patches! I’ve reviewed 0001 - 0003 so far; here are my
comments.

0001:
The code looks good to me. However, I feel a bit uneasy about not
seeing a test case for the additional subplan origin display added in
pg_overexplain. Maybe we could add the following test cases to
exercise that code:

-- should show "Subplan: sub"
EXPLAIN (RANGE_TABLE, COSTS OFF)
SELECT * FROM vegetables v,
(SELECT * FROM vegetables WHERE genus = 'daucus' OFFSET 0) sub;

-- should show "Subplan: unnamed_subquery"
EXPLAIN (RANGE_TABLE, COSTS OFF)
SELECT * FROM vegetables v,
(SELECT * FROM vegetables WHERE genus = 'daucus' OFFSET 0);

0002:
Looks good to me.

0003:
I see code like this:

@@ -2232,6 +2251,11 @@ accumulate_append_subpath(Path *path, List
**subpaths, List **special_subpaths)
if (!apath->path.parallel_aware || apath->first_partial_path == 0)
{
*subpaths = list_concat(*subpaths, apath->subpaths);
+ *child_append_relid_sets =
+ lappend(*child_append_relid_sets, path->parent->relids);
+ *child_append_relid_sets =
+ list_concat(*child_append_relid_sets,
+ apath->child_append_relid_sets);

in accumulate_append_subpath(), but in get_singleton_append_subpath()
there are only calls to lappend() and no list_concat(). Is that
intentional? Do we also want to concatenate the newly pulled up
child_append_relid_sets with the existing ones in
get_singleton_append_subpath()?

In add_paths_to_append_rel():

@@ -1785,13 +1790,16 @@ add_paths_to_append_rel(PlannerInfo *root,
RelOptInfo *rel,
{
Path *path = (Path *) lfirst(l);
AppendPath *appendpath;
+ AppendPathInput append = {0};
+
+ append.partial_subpaths = list_make1(path);
+ append.child_append_relid_sets = list_make1(rel->relids);

Could you help me understand why we need to populate
append.child_append_relid_sets here? I don’t see this child rel being
pulled up at this point.

0004:
I’ve only read through the README and documentation so far; I’ll
continue reviewing the code in 0004.

Best,
Alex

--
Alexandra Wang
EDB: https://www.enterprisedb.com