Remove redundant grouping and DISTINCT columns.

Avoid explicitly grouping by columns that we know are redundant
for sorting, for example we need group by only one of x and y in
	SELECT ... WHERE x = y GROUP BY x, y
This comes up more often than you might think, as shown by the
changes in the regression tests.  It's nearly free to detect too,
since we are just piggybacking on the existing logic that detects
redundant pathkeys.  (In some of the existing plans that change,
it's visible that a sort step preceding the grouping step already
didn't bother to sort by the redundant column, making the old plan
a bit silly-looking.)

To do this, build processed_groupClause and processed_distinctClause
lists that omit any provably-redundant sort items, and consult those
not the originals where relevant.  This means that within the
planner, one should usually consult root->processed_groupClause or
root->processed_distinctClause if one wants to know which columns
are to be grouped on; but to check whether grouping or distinct-ing
is happening at all, check non-NIL-ness of parse->groupClause or
parse->distinctClause.  This is comparable to longstanding rules
about handling the HAVING clause, so I don't think it'll be a huge
maintenance problem.

nodeAgg.c also needs minor mods, because it's now possible to generate
AGG_PLAIN and AGG_SORTED Agg nodes with zero grouping columns.

Patch by me; thanks to Richard Guo and David Rowley for review.

Discussion: https://postgr.es/m/185315.1672179489@sss.pgh.pa.us

1 files changed, 8 insertions, 4 deletions

diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index 53f890bb483..50d23f922cc 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -3345,9 +3345,9 @@ estimate_path_cost_size(PlannerInfo *root,
 			}
 
 			/* Get number of grouping columns and possible number of groups */
-			numGroupCols = list_length(root->parse->groupClause);
+			numGroupCols = list_length(root->processed_groupClause);
 			numGroups = estimate_num_groups(root,
-											get_sortgrouplist_exprs(root->parse->groupClause,
+											get_sortgrouplist_exprs(root->processed_groupClause,
 																	fpinfo->grouped_tlist),
 											input_rows, NULL, NULL);
 
@@ -3636,7 +3636,7 @@ adjust_foreign_grouping_path_cost(PlannerInfo *root,
 	 * pathkeys, adjust the costs with that function.  Otherwise, adjust the
 	 * costs by applying the same heuristic as for the scan or join case.
 	 */
-	if (!grouping_is_sortable(root->parse->groupClause) ||
+	if (!grouping_is_sortable(root->processed_groupClause) ||
 		!pathkeys_contained_in(pathkeys, root->group_pathkeys))
 	{
 		Path		sort_path;	/* dummy for result of cost_sort */
@@ -6415,7 +6415,11 @@ foreign_grouping_ok(PlannerInfo *root, RelOptInfo *grouped_rel,
 		Index		sgref = get_pathtarget_sortgroupref(grouping_target, i);
 		ListCell   *l;
 
-		/* Check whether this expression is part of GROUP BY clause */
+		/*
+		 * Check whether this expression is part of GROUP BY clause.  Note we
+		 * check the whole GROUP BY clause not just processed_groupClause,
+		 * because we will ship all of it, cf. appendGroupByClause.
+		 */
 		if (sgref && get_sortgroupref_clause_noerr(sgref, query->groupClause))
 		{
 			TargetEntry *tle;