Fix UNION planner estimate_num_groups with varno==0

03d40e4b5 added code to provide better row estimates for when a UNION
query ended up only with a single child due to other children being
found to be dummy rels.  In that case, ordinarily it would be ok to call
estimate_num_groups() on the targetlist of the only child path, however
that's not safe to do if the UNION child is the result of some other set
operation as we generate targetlists containing Vars with varno==0 for
those, which estimate_num_groups() can't handle.  This could lead to:

ERROR:  XX000: no relation entry for relid 0

Fix this by avoiding doing this when the only child is the result of
another set operation.  In that case we'll fall back on the
assume-all-rows-are-unique method.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/cfbc99e5-9d44-4806-ba3c-f36b57a85e21@gmail.com

1 files changed, 13 insertions, 10 deletions

diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index 72539545656..f528f096a56 100644
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -901,19 +901,22 @@ generate_union_paths(SetOperationStmt *op, PlannerInfo *root,
 		double		dNumGroups;
 		bool		can_sort = grouping_is_sortable(groupList);
 		bool		can_hash = grouping_is_hashable(groupList);
+		Path	   *first_path = linitial(cheapest_pathlist);
 
-		if (list_length(cheapest_pathlist) == 1)
+		/*
+		 * Estimate the number of UNION output rows.  In the case when only a
+		 * single UNION child remains, we can use estimate_num_groups() on
+		 * that child.  We must be careful not to do this when that child is
+		 * the result of some other set operation as the targetlist will
+		 * contain Vars with varno==0, which estimate_num_groups() wouldn't
+		 * like.
+		 */
+		if (list_length(cheapest_pathlist) == 1 &&
+			first_path->parent->reloptkind != RELOPT_UPPER_REL)
 		{
-			Path	   *path = linitial(cheapest_pathlist);
-
-			/*
-			 * In the case where only one union child remains due to the
-			 * detection of one or more dummy union children, obtain an
-			 * estimate on the surviving child directly.
-			 */
 			dNumGroups = estimate_num_groups(root,
-											 path->pathtarget->exprs,
-											 path->rows,
+											 first_path->pathtarget->exprs,
+											 first_path->rows,
 											 NULL,
 											 NULL);
 		}