Fix assertion failure in generate_orderedappend_paths()

In generate_orderedappend_paths(), there is an assumption that a child
relation's row estimate is always greater than zero.  There is an
Assert verifying this assumption, and the estimate is also used to
convert an absolute tuple count into a fraction.

However, this assumption is not always valid -- for example, upper
relations can have their row estimates unset, resulting in a value of
zero.  This can cause an assertion failure in debug builds or lead to
the tuple fraction being computed as infinity in production builds.

To fix, use the row estimate from the cheapest_total path to compute
the tuple fraction.  The row estimate in this path should already have
been forced to a valid value.

In passing, update the comment for generate_orderedappend_paths() to
note that the function also considers the cheapest-fractional case
when not all tuples need to be retrieved.  That is, it collects all
the cheapest fractional paths and builds an ordered append path for
each interesting ordering.

Backpatch to v18, where this issue was introduced.

Bug: #19102
Reported-by: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
Author: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Kuntal Ghosh <kuntalghosh.2007@gmail.com>
Reviewed-by: Andrei Lepikhov <lepihov@gmail.com>
Discussion: https://postgr.es/m/19102-93480667e1200169@postgresql.org
Backpatch-through: 18

src/backend/optimizer/path/allpaths.c

@@ -1810,9 +1810,11 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
  * We generate a path for each ordering (pathkey list) appearing in
  * all_child_pathkeys.
  *
- * We consider both cheapest-startup and cheapest-total cases, ie, for each
- * interesting ordering, collect all the cheapest startup subpaths and all the
- * cheapest total paths, and build a suitable path for each case.
+ * We consider the cheapest-startup and cheapest-total cases, and also the
+ * cheapest-fractional case when not all tuples need to be retrieved.  For each
+ * interesting ordering, we collect all the cheapest startup subpaths, all the
+ * cheapest total paths, and, if applicable, all the cheapest fractional paths,
+ * and build a suitable path for each case.
  *
  * We don't currently generate any parameterized ordered paths here.  While
  * it would not take much more code here to do so, it's very unclear that it
@@ -1977,14 +1979,18 @@ generate_orderedappend_paths(PlannerInfo *root, RelOptInfo *rel,
 				double		path_fraction = root->tuple_fraction;
 
 				/*
-				 * Merge Append considers only live children relations.  Dummy
-				 * relations must be filtered out before.
+				 * We should not have a dummy child relation here.  However,
+				 * we cannot use childrel->rows to compute the tuple fraction,
+				 * as childrel can be an upper relation with an unset row
+				 * estimate.  Instead, we use the row estimate from the
+				 * cheapest_total path, which should already have been forced
+				 * to a sane value.
 				 */
-				Assert(childrel->rows > 0);
+				Assert(cheapest_total->rows > 0);
 
 				/* Convert absolute limit to a path fraction */
 				if (path_fraction >= 1.0)
-					path_fraction /= childrel->rows;
+					path_fraction /= cheapest_total->rows;
 
 				cheapest_fractional =
 					get_cheapest_fractional_path_for_pathkeys(childrel->pathlist,

src/test/regress/expected/partition_aggregate.out

@@ -339,6 +339,37 @@ SELECT a FROM pagg_tab WHERE a < 3 GROUP BY a ORDER BY 1;
  2
 (3 rows)
 
+-- Test partitionwise aggregation with ordered append path built from fractional paths
+EXPLAIN (COSTS OFF)
+SELECT count(*) FROM pagg_tab GROUP BY c ORDER BY c LIMIT 1;
+                         QUERY PLAN                         
+------------------------------------------------------------
+ Limit
+   ->  Merge Append
+         Sort Key: pagg_tab.c
+         ->  GroupAggregate
+               Group Key: pagg_tab.c
+               ->  Sort
+                     Sort Key: pagg_tab.c
+                     ->  Seq Scan on pagg_tab_p1 pagg_tab
+         ->  GroupAggregate
+               Group Key: pagg_tab_1.c
+               ->  Sort
+                     Sort Key: pagg_tab_1.c
+                     ->  Seq Scan on pagg_tab_p2 pagg_tab_1
+         ->  GroupAggregate
+               Group Key: pagg_tab_2.c
+               ->  Sort
+                     Sort Key: pagg_tab_2.c
+                     ->  Seq Scan on pagg_tab_p3 pagg_tab_2
+(18 rows)
+
+SELECT count(*) FROM pagg_tab GROUP BY c ORDER BY c LIMIT 1;
+ count 
+-------
+   250
+(1 row)
+
 RESET enable_hashagg;
 -- ROLLUP, partitionwise aggregation does not apply
 EXPLAIN (COSTS OFF)

src/test/regress/sql/partition_aggregate.sql

@@ -76,6 +76,11 @@ EXPLAIN (COSTS OFF)
 SELECT a FROM pagg_tab WHERE a < 3 GROUP BY a ORDER BY 1;
 SELECT a FROM pagg_tab WHERE a < 3 GROUP BY a ORDER BY 1;
 
+-- Test partitionwise aggregation with ordered append path built from fractional paths
+EXPLAIN (COSTS OFF)
+SELECT count(*) FROM pagg_tab GROUP BY c ORDER BY c LIMIT 1;
+SELECT count(*) FROM pagg_tab GROUP BY c ORDER BY c LIMIT 1;
+
 RESET enable_hashagg;
 
 -- ROLLUP, partitionwise aggregation does not apply